Must I believe someone who makes a false oath, even though I know he is lying?

It is not permissible to believe a liar, as long as you know for certain that he is a liar, even if he swears by it; for the Prophet, peace and blessings be upon him, said: "It is sufficient falsehood for a man to narrate everything he hears." Lying is forbidden, and it becomes even more forbidden and a greater sin if its perpetrator swears by it. This is called a false oath (al-yamin al-ghamūs), and it is one of the major sins.