The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wa'l-Adab)
Muslim :: Book 32 : Hadith 6190
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: Let him be humbled, let him be humbled. It was said: Allah's Messenger, who is he? He said. He who finds his parents in old age, either one or both of them, and does not enter Paradise.