From Sahih Bukhari (Hadith 6919):
Narrated Abu Bakra: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)
Reference: Bukhari 6919