Narrated Abdullah رضي الله عنه (May Allah be pleased with him): I asked the Prophet صلى الله عليه وسلم (Peace be upon him) "Which deed is the dearest to Allah?" He replied, "To offer the prayers at their early stated fixed times." I asked, "What is the next (in goodness)?" He replied, "To be good and dutiful to your parents"

[Sahi Bukhari, Hadith no. 527]

Abu Huraira رضي الله عنه (May Allah be pleased with him) reported Allah's Messenger (Peace be upon him) as saying: Let him be humbled into dust; let him be humbled into dust. It was said: Allah's Messenger, who is he? He said: He who sees either of his parents during their old age or he sees both of them, but he does not enter Paradise.

[Sahi Muslim, Hadith no. 6510]

Narrated Abu Bakra رضي الله عنه (May Allah be pleased with him): Allah's Messenger صلى الله عليه وسلم (Peace be upon him) said thrice, "Shall I not inform you of the biggest of the great sins?" We said, "Yes, O Allahs Apostle" He said, "To join partners in worship with Allah: to be undutiful to ones parents."

[Sahi Bukhari, Hadith no. 5976]