Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1657
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: Allah descends every night to the lowest heaven when one-third of the first part of the night is over and says: I am the Lord; I am the Lord: who is there to supplicate Me so that I answer him? Who is there to beg of Me so that I grant him? Who is there to beg forgiveness from Me so that I forgive him? He continues like this till the day breaks.