Sufyan ibn Asid Al-Hadrami Radiyallahu 'anhu narrates: I heard Rasullullah Sallallahu 'alaihi wasallam saying:
It is great unfaithfulness that you say something to your brother, and he believes you while you are lying.
(Abu Dawud Hadith 4971)
Note: This means that though lying is a serious sin, but in certain situations its seriousness is intensified. For instance, a man trusts you and you take undue advantage of this trust by lying and deceiving him.
