Musnad Abu Ya'la al-Mawsili 160:181

'Uqbah ibn Mukram narrated to us, Yunus narrated to us, from Muhammad ibn Ishaq, from Sa'id ibn Abi Sa'id, from Abu Hurayrah, and from his uncle 'Abd al-Rahman ibn Yasar, from 'Ubayd Allah ibn Abi Rafi', from 'Ali, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Were it not that I would burden my Ummah, I would have delayed the late 'Isha' prayer until the third of the night. For when the first third of the night passes, Allah (the Mighty and Majestic) descends to the lowest heaven, and He remains there until the dawn breaks. He says: 'Is there any repentant one? Is there any asker who will be given? Is there any supplicant who will be answered? Is there any sinner who will seek forgiveness, so that he may be forgiven? Is there any sick person who will seek healing, so that he may be healed?'"