Musnad Ahmad 5:403

Ya'qub narrated to us, my father narrated to us, from Muhammad ibn Ishaq, Sa'id ibn Abi Sa'id al-Maqburi narrated to me, from 'Ata', the freed slave of Umm Sabiyah, from Abu Hurayrah, who said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "Were it not that I would burden my nation, I would have commanded them to use the siwak at every prayer, and I would have delayed the final Isha prayer until the first third of the night. For when the first third of the night has passed, Allah the Exalted descends to the lowest heaven, and He remains there until the dawn breaks. Then a caller says: 'Is there any petitioner who will be given? Is there any supplicant who will be answered? Is there any sick person seeking healing who will be healed? Is there any sinner seeking forgiveness who will be forgiven?'"