Takhrij Hadith 1574: "I witnessed Ali when a…"

I witnessed Ali when a mount was brought to him so that he could ride it. When he put his foot in the stirrup, he said: In the name of Allah. When he settled himself on its back, he recited: Exalted is He who has subjected this to us, and we could not have otherwise subdued it. And indeed we, to our Lord, will surely return. He then recited: Praise be to Allah three times, and then: Allah is the Greatest three times. Then he said: Exalted are You above any imperfection. I have wronged myself, so forgive me, for none but You can forgive sins, and then he laughed. It was asked: O Commander of the Believers, what made you laugh?. He replied: I saw the Prophet (may Allah's peace and blessings be upon him) doing as I have done then he laughed. I asked him: O Messenger of Allah, what made you laugh? He said: Your Lord, the Exalted, is pleased when His slave seeks His forgiveness; he (the slave) knows that none can forgive sins except Allah.