Ibn Majah

1338. Commentary:

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.

Hadith Commentary: Benefits and Issues: ➊
It is established from this hadith that it was the practice of the Prophet sallallahu alayhi wa sallam that immediately after turning for salam (after saying "Allahu Akbar"), he would say "Astaghfirullah" three times.
Because this is the height of servitude and worship that even after an act of worship like prayer, one considers oneself at fault, deficient, and incapable of fulfilling the right of worship, and seeks forgiveness and pardon from Allah, and in reality, one should admit that the right has not been fulfilled.

➋
Regarding the brief supplication of Thawban radi Allahu anhu, which he narrated from the Messenger of Allah sallallahu alayhi wa sallam, nowadays some words are commonly added on one's own, such as:
(Wa ilayka yarji‘u as-salaam fa hayyina rabbana bis-salaam wa adkhilna daraka dar as-salaam)
Allamah Sa‘idi writes:
"In the noble hadith, the words of supplication and remembrance that have been transmitted, it is not correct to add, subtract, or alter them from oneself.
The Messenger of Allah sallallahu alayhi wa sallam taught a supplication to al-Bara’ bin ‘Azib radi Allahu anhu, in which the words were:
(Wa nabiyyika alladhi arsalta)
When al-Bara’ radi Allahu anhu repeated these words back to him, he recited:
(Wa birasulika alladhi arsalta)
He sallallahu alayhi wa sallam said:
La,
No.
(Wa nabiyyika alladhi arsalta)
Recite only the words that I have taught you.
(Vol: 2, p: 192)
He then quotes the statements of Hafiz Ibn Hajar rahimahullah and Allamah ‘Ayni rahimahullah, the meaning of which is:
"The words of remembrance (adhkar) are divinely prescribed (tawfiqi) in the specification of the wording and the amount of reward,
(one must adhere to what has been transmitted),
because sometimes there is a secret in one word that is not found in another word of similar meaning."