Abu Dawud Book 2, Hadith Number 1310.
Chapter : Which part of night is better for prayer.
Narated By Abu Hurairah : The Apostle of Allah (PBUH) said: Our Lord who is blessed and exalted descends every night to the lowest heaven when one-third of the night remains, and says: Who supplicates Me so that I may answer him? Who asks Me so that I may give to him? Who asks My forgiveness so that I may forgive him?