Musannaf Ibn Abi Shayba 21:767

'Affan narrated to us, he said: Hammad ibn Salamah narrated to us, he said: We were in a place that beasts could not pass through, so I stood up while I was reciting these verses: "The Forgiver of sin, the Acceptor of repentance, the Severe in punishment." He said: Then an old man passed by on a grey mule. He said: "Say: O Forgiver of sin, forgive my sin; O Acceptor of repentance, accept my repentance; O Severe in punishment, pardon my punishment; O Possessor of bounty, bestow upon me with good." He said: So I said it, then I looked, and I did not see him.