The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1658
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: When half of the night or two-third of it is over. Allah, the Blessed and the Exalted, descends to the lowest heaven and says: Is there any beggar, so that he be given? Is there any supplicator so that he be answered? Is there any beggar of forgiveness so that he be forgiven? (And Allah continues it saying) till it is daybreak.