Musnad Ahmad 5:191

Yazid narrated to us, Sharik ibn Abdullah informed us, from Abu Ishaq, from Ali ibn Rabiah, who said: I saw Ali, and a beast was brought to him for him to ride. When he placed his foot in the stirrup, he said: "In the name of Allah." When he settled upon it, 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, to our Lord we will surely return." Then he praised Allah three times, and magnified Allah three times, then said: "Exalted are You, there is no god but You. I have wronged my soul, so forgive me." Then he laughed. I said: "Why did you laugh, O Commander of the Faithful?" He said: "I saw the Messenger of Allah, peace and blessings of Allah be upon him, do as I did, then he laughed. I said: 'Why did you laugh, O Messenger of Allah?' He said: 'The Lord is amazed at His servant when he says: My Lord, forgive me; and He says: My servant knows that no one forgives sins except Me.'"