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 6189
Abu Huraira reported Allah's Apostle (may 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.