Sahih Bukhari Volume 8, Book 73, Hadith Number 7.

Narated By Abu Bakra : Allah’s Apostle said thrice, “Shall I not inform you of the biggest of the great sins?” We said, “Yes, O Allah’s Apostle” He said, “To join partners in worship with Allah: to be undutiful to one’s parents.” The Prophet sat up after he had been reclining and added, “And I warn you against giving forged statement and a false witness; I warn you against giving a forged statement and a false witness.” The Prophet kept on saying that warning till we thought that he would not stop.