Musnad Ahmad 5:367

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Abu Ishaq, from 'Ali ibn Rabi'ah—he said once: 'Abd al-Razzaq said—and most of the time he says: "One who witnessed 'Ali informed me when he mounted. 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.' Then he said: '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.' Then he praised [Allah] three times, and magnified [Allah] three times. Then he said: 'O Allah, there is no god but You. I have wronged my soul, so forgive me, for indeed no one forgives sins except You.' Then he laughed. It was said: 'What makes you laugh, O Commander of the Faithful?' He said: 'I saw the Prophet, peace and blessings of Allah be upon him, do as I did, and say as I said. Then he laughed. We said: 'What makes you laugh, O Prophet of Allah?' He said: 'The servant—or he said: I am amazed at the servant—when he says: There is no god but You, I have wronged my soul, so forgive me, for indeed no one forgives sins except You; he knows that no one forgives sins except Him.'"