Musnad Ahmad 30:2463

Yahya narrated to us, from Ubaydullah, who said: Sa’id informed me, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him), who said: "Were it not that I would burden my nation, I would have commanded them to use the siwak with every ablution, and I would have delayed the Isha prayer until a third of the night, or half of the night." "Then, when a third of the night, or half of the night, has passed, He descends to the lowest heaven and says: 'Is there any requester, that I may give him? Is there any seeker of forgiveness, that I may forgive him? Is there any repentant one, that I may accept his repentance? Is there any supplicant, that I may answer him?'"