Abu Bakra:
The Prophet (ﷺ) said thrice, "Should I inform you out the greatest of the great sins?" They said, "Yes, O
Allah's Messenger (ﷺ)!" He said, "To join others in worship with Allah and to be undutiful to one's parents."
The Prophet (ﷺ) 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)