Musnad Ahmad 30:474

Abu Kamil narrated to us, Ibrahim narrated to us, Ibn Shihab narrated to us, from al-Agharr and Abu Salamah ibn Abd al-Rahman, from Abu Hurairah, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Our Lord, blessed be His name, descends every night, when the last third of the night remains, to the lowest heaven, and He says: 'Who is calling upon Me, that I may answer him? Who is asking of Me, that I may give to him? Who is seeking My forgiveness, that I may forgive him?' until the dawn breaks." For this reason, they used to prefer the prayer of the last part of the night over the prayer of the first part of it.