Narrated Abu Bakra: The Prophet said thrice, Should I inform you out the greatest of the great sins? They said, Yes, O Allah's Apostle! He said, To join others in worship with Allah and to be undutiful to one's parents. The Prophet then sat up after he had been reclining (on a pillow) and said, And I warn you against giving a false witness, and he kept on saying that warning till we thought he would not stop. (See Hadith No. 7, Vol. 8)