Musnad Abu Ya'la al-Mawsili 3:326

Abu Khaythama narrated to us, Jarir narrated to us, from Mansur, from Abu Ishaq, from Ali ibn Rabi'ah al-Asadi, who said: I saw Ali, and a beast was brought to him. He placed his foot in the stirrup and said, "In the name of Allah." When he was settled upon it, 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, to our Lord, will [surely] return" (Az-Zukhruf). Then he said "Allahu Akbar" three times and "Alhamdulillah" three times. Then he said, "Glory be to You, I have wronged my soul, so forgive me, for indeed no one forgives sins except You." Then he laughed. I said, "What made you laugh?" He said, "The Messenger of Allah (peace and blessings of Allah be upon him) said one day just as I said, then he laughed. I said, 'What made you laugh, O Messenger of Allah?' He said, 'Our Lord is amazed at the saying of His servant: "Glory be to You, I have wronged my soul, so forgive me, for indeed no one forgives sins except You." He said: My servant knows that he has a Lord who forgives sins.'"