“When the Messenger of Allah — may Allah’s peace and blessings be upon him — finished his prayer, he would seek forgiveness three times and say: ‘O Allah, You are al-Salām and from You is all peace; blessed are You, O Possessor of majesty and honor.’”
Al-Walīd said: “I said to al-Awzāʿī: How is the seeking of forgiveness?” He replied: “You say: Astaghfirullāh, Astaghfirullāh.”

Reported by Muslim (no. 591), from the narration of Thawbān — may Allah be pleased with him.

Brief Explanation of the Hadith

A servant should not be deceived by his worship, for no matter how much effort he puts into it, Satan will still have a share in it. Therefore, seeking forgiveness after it has been legislated—to make up for shortcomings and to mend any deficiencies. As in the report of Thawbān — may Allah be pleased with him — the Prophet — may Allah’s peace and blessings be upon him — when he concluded his prayer, would say: Astaghfirullāh, Astaghfirullāh, Astaghfirullāh—out of humility before Allah, the Mighty and Majestic, manifesting the perfection of servitude, and teaching the Ummah.
It was from the guidance of the Prophet — may Allah’s peace and blessings be upon him — to follow seeking forgiveness with supplication to Allah and earnest entreaty: affirming that He is free from all deficiency, that from Him safety in one’s religion and worldly affairs is sought, then acknowledging His majesty and the greatness of His status, and that to Him belong all greatness, sovereignty, generosity, and bounty.
The hadith demonstrates the humility of the Prophet — may Allah’s peace and blessings be upon him — and his manifestation of servitude to Allah, the Exalted, as he would seek forgiveness from his Lord even though Allah had forgiven him his past and future sins.
It also shows the legislation of seeking forgiveness after acts of worship, and that seeking forgiveness is not limited to sins; rather, it is also prescribed to make up for any shortcomings or deficiencies that may occur in one’s acts of obedience.