Many of the early scholars held the firm opinion that to act upon, or derive rulings from a hadeeth which has been declared to be weak, by the scholars of hadeeth is unacceptable. Their reasoning being that Islaam has no need of anything weak, and the authentic material of Islaam will suffice for all time, the da’eef hadeeth amounting only to a conjecture which has the possibility of being correct.
I quote from the introduction of ‘The Prophets Prayer Described’ of Shaykh al-Albaanee,
|“…this is because I hold that the authentic ahaadeeth are sufficient, leaving no need for anything weak, for the latter does not amount to anything except dhann (conjecture, suspicion), and incorrect conjecture at that; as the Exalted says:|
|“…and conjecture is of no use against the truth.”|
And now the following quotes will show the position of the early Scholars of Hadeeth on this issue:
Imaam ash-Shaafi’ee  says in his ‘Risaalah’ (394-403: #1090-1105),
|‘[#1090] Surely, the greatest of liars is he who ascribes to me that which I did not say, and who claims to have dreamt what he did not dream, and who claims that he is the son of someone other than his own father.|
|[#1091] Whoever ascribes to me that which I did not say, will surely have to occupy his seat in the fire [of hell]|
|[#1092] Surely, whoever tells untruths about me, will have a house built for him in the fire [of hell].|
|[#1093] Whoever tells untruths about me is surely seeking for himself a resting place in the fire [of hell]. The Messenger of Allah began to say that while he was wiping the ground with his hand.|
|[#1094] From Abu Hurayra, “You may report about the Children of Israel and there is no blame (haraj). Report about (/from) me, but do not tell untruths about me.”|
|[#1095] This is the most emphatic hadeeth ever transmitted from the Messenger of Allah on this matter. We have relied on it as well as on others (ahaadeeth or evidences) in not accepting any report (hadeethan) except from a trustworthy [transmitter], and that we know the truthfulness of those who transmitted the hadeeth since it was begun till its end is reached.|
|[#1096] If someone would say: What evidence is there in this hadeeth for what you have stated?|
|[#1097] It would be said: Knowledge surely has made it certain that the Prophet would never, in any circumstances, order anyone to lie about the Children of Israel, nor about anyone else. So when he has permitted reporting about (al-hadeetha ‘an) the Children of Israel, it was not accepting untruthfulness about the Children of Israel that he has permitted, but he only has permitted accepting that from whom reported it, whose truthfulness or untruthfulness is not known.”|
Imaam Muslim states in the introduction to his saheeh, under the chapter heading,
|“the weak ahaadeeth are to be discarded and only authentic ahaadeeth are to be narrated,”|
|“To proceed, may Allah have mercy upon you. If it were not from the evil practice that we have seen from many who take upon themselves the position of Muhaddith, in their leaving the obligation to discard the weak ahaadeeth and munkar narrations and to suffice with only the authentic ahaadeeth – well known and transmitted from reliable narrators, well known for their truthfulness and trustworthiness. After knowing and admitting with their tongues that much of what they fling at the ignorant is to be rejected and is transmitted by unsatisfactory narrators whose narrations are censured by the scholars of hadeeth like Maalik, Yahya bin Sa’eed al-Qattaan and others…..|
|And know may Allah have mercy upon you, that what is obligatory upon everyone who is able to distinguish between authentic and weak narrations and between the suspect and reliable narrators, is that he should narrate therefrom except that known to be authentic and have trustworthy narrators…”|
Imaam ibn Rajab al-Hanbali  says,
|‘and it is clear from what Muslim mentions in the introduction to his book (i.e. Saheeh Muslim), that it is necessary that the ahaadeeth to do with Targheeb wat Tarheeb (encouragement and discouragement) are not narrated except from those that ahkaam (rules and regulations) are narrated [meaning the authentic ahaadeeth]’|
al-Allaamah Jamaal ud-Deen al-Qaasimee narrates from a group of the Imaams of hadeeth that they did not accept acting by a weak hadeeth at all, like ibn Ma’een, al-Bukhari, Muslim, Abu Bakr ibn al-Arabi al-Maaliki, ibn Hazm and others. 
Abu Bakr ibn al-Arabi said, while commenting on the hadeeth, the halaal is clear and the haraam is clear”,
|“…. What I have [as the reason] regarding that, and Allah knows best, is that which we have transmitted from Ahmad ibn Hanbal that he permits weak ahaadeeth regarding al-wara` (abstaining from doubtful matters). May Allah be pleased with al-Bukhari who did not see for the heart to hold to, nor for the religion to be connected through–nothing except the authentic [ahaadeeth], and that is our position. If we were to incline to the position of Ahmad; then holding to da’eef ahaadeeth cannot be [accepted] except in lessons/admonishment which soften the heart, but as for the basis (usul) there is no way to [accept] that.” |
Ibn Hazm  says in ‘al-Milal’, “and it is not permissible with us that we say as these ahaadeeth say (i.e. those weak and fabricated narrations), or to trust in them, or to take anything from them.”
ibn Taymiyyah says, ‘and it is not permissible to rely in the Sharee’ah upon da’eef ahaadeeth which are not saheeh or hasan. But Ahmad bin Hanbal and other scholars considered it permissible to report with regards to Fadaa’il al-A’amaal (rewards and excellences of actions) that which they did not know to be affirmed, when it is known that it is not a lie. And that is because when the action is known to be legislated with a Sharee’ah evidence, it is possible that the reward be a fact. And not one of the Imaams said that he considered it to be permissible to make something obligatory or recommended based upon a da’eef hadeeth.’
Then ibn Taymiyyah says, ‘and Ahmad bin Hanbal or others like him from the Imaams did not rely upon this type of ahaadeeth in the Sharee’ah. And the one who relates from Ahmad that he used to rely upon the weak ahaadeeth, which are not saheeh or hasan, has erred.’
So the narrations from him that he would act upon a da’eef hadeeth when there was nothing else present in the texts on that subject, or nothing that contradicted that da’eef hadeeth, does not mean that Imaam Ahmad used them as proof in the Sharee’ah. Allah knows best.
al-Allaamah Ahmad Shaakir says,‘and as for what Imaam Ahmad bin Hanbal and Abdurrahmaan bin Mahdi, and Abdullaah bin al-Mubaarak said, “when it is narrated to us pertaining to Halaal wal Haraam we are strict. And when it is narrated to us pertaining to Fadaa’il al-A’maal (the rewards and excellence of actions) then we are lenient.” – then they mean, according to what I find to be most convincing – and Allah knows best – that the leniency was in their taking the hasan hadeeth, that which does not reach the level of saheeh. Because the convention of distinguishing between the Saheeh and Hasan was not present at their time…rather many of the early scholars did not describe a hadeeth except by it being saheeh or da’eef only.’
So what is clear is that the term hasan was not present at that time, and a great deal of what these scholars used to narrate in terms of fadaa’il al-amaal was of the level of what the later scholars called hasan.
So the opinion from the above mentioned scholars is to leave acting by the weak hadeeth in totality, except where there is a consensus of the Islamic scholars on the issue at hand, and Allah knows best. And as for the claims of some that Imaam Ahmad amongst others of the early scholars allowed weak ahaadeeth to be used in Sharee’ah rulings then that has no firm basis as mentioned above. 
And especially in this day and age, when so many innovations and misunderstandings about Religion are present, many of them having their roots in these da’eef ahaadeeth, it becomes even more essential to narrate only authentic ahaadeeth as part of the process of purifying the understanding of the Religion.
The preceding was with regards to acting upon the da’eef hadeeth, as regards to using the da’eef hadeeth in certain Islamic sciences like in the Hadeeth science in which the weak ahaadeeth are used to support or strengthen other ahaadeeth, then this has been done and is being done by all of the Scholars of Hadeeth.
For those that follow the opinion that acting upon a da’eef ahaadeeth is permissible, it would be good to mention the three conditions for acting upon a da’eef ahaadeeth as laid out by Ibn Hajr al-Asqalaanee:
1) Upon that which they all agree, that it should not be very weak so that it excludes that only narrated by a liar, one accused of lying and one who makes serious mistakes.
2) That is falls under a general proof already present – which excludes that which is invented having no basis.
3) That acting upon it the person does not think that it is something established – in order that he does not attribute to the Prophet what he did not say.
We can see from these condition the following: The first principle lays out the obligation to make known the weak ahaadeeth from the authentic, even in Fadaa’il al-Amaal. Something which many people who follow this opinion do not do, not only that but many of the scholars who follow this opinion today are not even capable of discerning whether the hadeeth they are quoting contains the types of weaknesses indicated above!
The second principle establishes that in reality the person is not acting by the weak hadeeth but rather by the general proof already present. 
1.He is the Mujtahid Imaam and the Mujaddid of his time, Muhammad bin Idrees ash-Shaafi’ee. He studied under a galaxy of prominent Imaams, amongst them Imaam Maalik.
2.’Sharh at-Tirmidhee’ (2/112).He is the exemplary Imaam and great Mujtahid scholar, ibn Rajab al Hanbali, a student of both ibn Taymiyyah and ibn al-Qayyim amongst others.
3.’Qawaa’id al-Hadeeth’ (pg. 113) of al-Qaasimee.
4.’Aridat al-Ahwadhee Sharh Sunan at-Tirmidhee’ (5/201)
5.Quoted from Abu Ghuddas introduction to al-Muhasibi’s ‘Risalah al-Mustarshideen’ (pp58) where he states that he has abridged the quote from ibn al-Arabi.
6.He is the great Imaam who championed the Dhaahiree School of thought and wrote many invaluable treatise, amongst them his ‘Muhalla’ in usul al-fiqh, and ‘Milal wa Nahl’ on different sects. He died in the year????
7.’al-Qaa’idah al-Jaleelah’ (pg.82) of ibn Taymiyyah
8.’al-Baa’ith al-Hatheeth’ (pg.101) of Ahmad Shaakir.
9.So what would the author of ‘al-Albani Unveiled’ say about the position of all these scholars, would he label them all as ignorant as well, as he has done in his amazing ‘scholarly’ work?!
10.As mentioned by his student as-Sakhaawi in his
11.The quotes are taken from the introductions of ‘Saheeh al-Jaami as-Sagheer’, ‘Tamaam al-Minna’, ‘Silsilah ad-Da’eefah’ (Vol. 1) of al-Albaanee, with the exception of the quotes of Imaam ash-Shaafi’ee and ibn al-Arabi.