The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1660
Abu Sa'id and Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: Allah waits till when one-third of the first part of the night is over; He descends to the lowest heaven and says: It there any supplicator of forgiveness? Is there any penitant? Is there any petitioner (for mercy and favour)? Is there any solicitor? -till it is daybreak.