SAHIH BUKHARI, BOOK 21 Prayer at Night (Tahajjud)
Volume 2, Book 21, Number 246:
Narrated Abu Huraira:
Allah's Apostle (p.b.u.h) said, "Our Lord, the Blessed, the Superior, comes every night down on the nearest Heaven to us when the last third of the night remains, saying: "Is there anyone to invoke Me, so that I may respond to invocation? Is there anyone to ask Me, so that I may grant him his request? Is there anyone seeking My forgiveness, so that I may forgive him?"
