The Prophet. said, "The biggest of the great sins are: To join others in worship with Allah, to be undutiful to one's parents, and to give a false witness." He repeated it thrice, or said, "....a false statement," and kept on repeating that warning till we wished he would stop saying it. (See Hadith No.7, Vol. 8)

Sahih al-Bukhari Book 88, Hadith 2