Sahih Bukhari Volume 009, Book 084, Hadith Number 054.
Narrated By 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. 8)