From Sahl ibn Sa'd (radiyallaahu 'anhu) who said that Allaah's Messenger (salallaahu 'alaihi wa'sallam) said:
Beware of sins which are treated as being minor, just like a people who encamp in the centre of a valley, so someone brings a stick of firewood and someone else brings a stick until they are therefore able to bake their bread. Likewise sins which are treated as being minor and for which the person is taken to account will destroy him.
So it is just as the noble companion 'Abdullah ibn Mas'ood said: The believer sees his sins as if he were sitting beneath a mountain which he feared was about to fall upon him, whereas the wicked sees his sins like a man who finds a fly settle upon his nose, so he does this (one of the narrators said: He brushes it away from his nose).
The Muslim should realise the greatness of hid Lord, the One free and far from all imperfections, and His tremendous Power and Might, and he should not think of the sin as being great or small! Rather he should think of it with regard to the One he is disobeying.
Those sins which are ignored by the weak hearted cause their destruction by removing eemaan and fear of Allaah from their hearts.
When a person regards sins and acts of disobedience as something slight the he is one who is oppressing his own soul, and that should never occur, since: