Narrated Abu Bakra (Radi Allah Anhu): The Prophet said thrice, "Should I inform you about the greatest of the great sins?" They said, "Yes, O Allah's Apostle!" He said, "To join others in worship with Allah (shirk) and to be undutiful to one's parents." The Prophet then sat up as before he was 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."

- Sahih Al-Bukhari