Musnad Ahmad 5:492

Waki’ narrated to us, from Isra’il, from Abu Ishaq, from ‘Ali ibn Rabi’ah, who said: I was riding behind ‘Ali, may Allah be pleased with him. When he placed his foot in the stirrup, he said: "In the name of Allah." When he sat firmly, he said: "Praise be to Allah. {Exalted is He who has subjected this to us, and we could not have [otherwise] subdued it. And indeed we will, to our Lord, return} [Az-Zukhruf: 13-14]." Abu Sa’id, the freed slave of Banu Hashim, said: "Then he praised Allah three times, and said 'Allah is the Greatest' three times." Then he said: "Exalted is Allah three times." Then he said: "There is no god but You." Then he returned to the narration of Waki’: "Exalted are You, I have wronged myself, so forgive me, for indeed no one forgives sins except You." Then he laughed. I said: "What makes you laugh?" He said: "I was riding behind the Messenger of Allah, may Allah bless him and grant him peace, and he did as you saw me do. Then he laughed. I said: 'O Messenger of Allah, what makes you laugh?' He said: 'Allah, Blessed and Exalted is He, said: My servant is amazed that he knows that no one forgives sins except Me.'"