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)

Volume 3, Book 48, Number 823:

Narrated Aisha: The Prophet performed the Tahajjud prayer in my house, and then he heard the voice of 'Abbas who was praying in the Mosque, and said, "O 'Aisha! Is this 'Abbad's voice?" I said, "Yes." He said, "O Allah! Be merciful to 'Abbas!"