(he informed us), (I heard), and (on the authority of) are used by the reporters of hadith. The first three usually indicate that the reporter heard in person from his shaikh. The mode can denote either hearing in person or through another reporter.
Tadlis (concealing) refers to an isnad where a reporter may have concealed the identity of his shaikh. Ibn al-Salah describes two types of Tadlis:
a) Tadlis al-Isnad; A person reports from his shaikh, whom he met, what he did not hear from him, or from a contemporary of his whom he did not meet, in such a way as to create the impression that he heard the hadith in person.
A mudallis normally uses the mode or to conceal the truth about the isnad.
b) Tadlis al-Shuyukh: The reporter does mention his shaikh by his usual name, but instead with a less well-known name, by-name, nickname or another less well-known identity, in order not to disclose his shaikh's identity.
Al-'Iraqi (d. 806), in his notes on Muqaddima Ibn al-Salah, adds a third type of Tadlis, known as Tadlis al-Taswiyya. To explain it, let us assume an isnad which contains a trustworthy shaikh reporting from a weak authority, who in turn reports from another trustworthy shaikh. Now, the reporter of this isnad omits the intermediate weak authority, leaving the isnad apparently consisting of reliable authorities. He plainly shows that he heard it from his shaikh but he uses the mode 'an to link his immediate shaikh with the next trustworthy one, thus omitting the intermediate authority. To an average reader, this isnad seems free of any doubt or discrepancy. This is known to have been practised by Baqiyya b. al-Walid, Walid b. Muslim, A'mash and al-Thauri. It is said to be the worst among the three kinds of Tadlis.
Ibn Hajar (d. 852) classifies the mudallisun into five categories in his essay Tabaqat al-Mudallisin:
a) Those who are known to do it occasionally, such as Yahya b. Sa'id al-Ansari.
b) Those who are accepted by the traditionists, either because of their good reputation and relatively few cases of Tadlis, e.g. al-Thauri, or because they reported from authentic authorities only, e.g. Ibn 'Uyaina.
c) Those who practised it a great deal. The traditionists have accepted only such ahadith from them which were reported with a clear mention of hearing directly. Among these are Abu Zubair al-Makki. Opinions differ regarding whether they are acceptable or not.
d) Similar to the previous category, but the traditionists agree that their ahadith are to be rejected unless they clearly admit of their hearing, such as Baqiyya b. al-Walid.
e) Those who are disparaged due to another reason apart from Tadlis; their ahadith are rejected, even though they admit of hearing them directly. Exempted from them are reporters such as Ibn Lahi'a, whose weakness is found to be of a lesser degree. Ibn Hajar gives the names of 152 such reporters.
Tadlis, especially of those in the last three categories, is so disliked that Shu'ba said, "Tadlis is the brother of lying" and "To commit adultery is more favourable to me than to report by way of Tadlis."
A musalsal (continued) isnad is one in which all the reporters as well as the Prophet use the same mode of transmission such as 'an, haddathana, etc., repeat any other additional statement or remark, or act in a particular manner while narrating the hadith.
Al-Hakim (d. 405) gives eight examples of such asanid, each having a different characteristic repeated feature:
a) use of the phrase sami'tu (I heard);
b) an expression such as "stand and pour water for me so that I may illustrate the way my shaikh performed ablution";
c) haddathana (he narrated to us);
d) an expression such as amarani (he commanded me);
e) an action such as holding one's beard;
f) illustrating by counting on five fingers;
g) an expression such as "I testify that ..."; and
h) interlocking the fingers.
Knowledge of musalsal helps in discounting the possibility of Tadlis.