Musnad Ahmad 572:19

'Affan narrated to us, he said: Hammad ibn Salamah narrated to us, 'Ali ibn Zayd narrated to us, from al-Hasan, from 'Uthman ibn Abi al-'As, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "A caller calls out every night: 'Is there any petitioner, so that I may give to him? Is there any seeker of forgiveness, so that I may forgive him? Is there any supplicant, so that I may answer him?'"