Musnad Ahmad 572:8

Rawh ibn ‘Ubadah narrated to us, Hammad ibn Salamah narrated to us, he said: ‘Ali ibn Zayd narrated to us, from al-Hasan, from ‘Uthman ibn Abi al-‘As, from the Prophet (peace and blessings of Allah be upon him), who said: "A caller calls out every night at an hour: Is there anyone supplicating so that I may answer him? Is there anyone asking so that I may give him? Is there anyone seeking forgiveness so that I may forgive him?"