Al-Matalib al-'Aliyah 3259 — Repentance and seeking forgiveness

It is narrated from Ali bin Rabi'ah (may Allah have mercy on him) that Sayyiduna Ali (may Allah be pleased with him) seated me behind him on his mount, then he went with me towards a graveyard, then he raised his blessed head towards the sky and recited this supplication: 'Allahumma ighfir li dhunubi innahu la yaghfiru al-dhunuba ahadun ghayruka' (O Allah, forgive me my sins; truly no one forgives sins except You). Then he turned towards me and began to smile, so I said: O Commander of the Believers! You are seeking forgiveness from your Lord and then you look at me and smile? He said: (In fact) the Messenger of Allah (peace and blessings be upon him) had seated me behind him on his mount, then he (peace and blessings be upon him) went with me to one side of the Harrah (the stony tract of Madinah), then he (peace and blessings be upon him) raised his blessed head towards the sky and supplicated: "'Allahumma ighfir li dhunubi innahu la yaghfiru al-dhunuba ahadun ghayruka' (O Allah, forgive me my sins, for no one forgives sins except You)." Then he (peace and blessings be upon him) turned towards me and smiled, so I said: O Messenger of Allah! You are both supplicating to your Lord for forgiveness and looking at me and smiling? So he (peace and blessings be upon him) said: "I smiled because my Lord, the Mighty and Majestic, smiled (in pleasure), for Allah Almighty marvels (being greatly pleased) at this quality of His servant: that he knows and is fully certain that no one besides Me forgives his sins."
This hadith is sahih (authentic) and its narrators are thiqat (trustworthy); Imam Abu Dawud and Imam al-Tirmidhi have also narrated it.