´Thawban, the freed slave of the Messenger of Allah (ﷺ), narrated that:` When he finished the prayer, the Messenger of Allah (ﷺ) would pray for forgiveness three times and say: 'Allahumma anta asalam, wa minka as-salam tabarakta ya dhal-jalali wal-ikram (O Allah, You are the source of eace (or the One free from all faults) and from You comes peace, blessed are You, O Possessor of Majesty and Honor)."
Hadith Referenceسنن نسائي / كتاب السهو / 1338
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:صحيح مسلم
➊ It is recommended (mustahabb) to seek forgiveness (istighfar) after concluding the prayer with salam.
➋ From this blessed hadith, the perfect humility and expression of servitude of the Noble Prophet (sallallahu alayhi wa sallam) before his Lord is established, despite the fact that Allah, the Exalted, had forgiven all his slips.
➌ A servant should not think that he is perfect in obedience; rather, he should consider that there is deficiency in his obedience, that he has not fulfilled the right of worship. He should continue striving to make up for this shortcoming through seeking forgiveness (istighfar).
➍ “Blessed are You” means that there is no deficiency with You; there is only abundance upon abundance. Or, wherever Your mention occurs, there is blessing.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 1338