Musannaf Ibn Abi Shayba 21:281

Al-Fadl ibn Dukayn narrated to us, from Isma'il ibn 'Abd al-Malik, from 'Ali ibn Rabi'ah, who said: 'Ali carried me behind him, then he traveled with me to the side of al-Harrah. Then he raised his head to the sky and said: "O Allah, forgive me my sins, for no one forgives sins except You." Then he turned to me and laughed. I said: "O Commander of the Faithful, your seeking forgiveness from your Lord and your turning to me while you are laughing?" He said: "The Messenger of Allah (may Allah bless him and grant him peace) carried me behind him, then he traveled with me to the side of al-Harrah. Then he raised his head to the sky and said: 'O Allah, forgive me my sins, for no one forgives sins except You.' Then he turned to me and laughed. I said: 'O Messenger of Allah, your seeking forgiveness from your Lord and your turning to me while you are laughing?'" He said: "I laughed at the laughter of my Lord, at His amazement for His servant, that he knows that no one forgives sins except Him."