Musnad Ahmad 30:3487

Ibn Abi ‘Adi narrated to us, from Muhammad ibn Ishaq, from Sa‘id ibn Abi Sa‘id al-Maqburi, from ‘Ata’, the freed slave of Umm Safiyyah—and Ya‘qub said: [the freed slave of] Sabiyyah, and it is the correct one—from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "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 last Isha prayer until the first third of the night. For when the first third of the night has passed, He descends to the lowest heaven until the break of dawn. A caller says: Is there any supplicant who will be answered? Is there any asker who will be given? Is there any sinner who will seek forgiveness so that he may be forgiven?"