Bukhari

Hadith Commentary:
(1)
The apparent wording of the hadith indicates that the Messenger of Allah (sallallahu alayhi wa sallam) would seek forgiveness and resolve to repent, regardless of the specific words used. As narrated from Ibn Umar (radi Allahu anhuma), we would count the following words of the Messenger of Allah (sallallahu alayhi wa sallam) one hundred times in a single gathering:
(Rabbighfir li wa tub 'alayya, innaka anta at-tawwab al-ghafur)
“My Allah! Forgive me and turn towards me in mercy. Indeed, You are the Most Forgiving, Most Accepting of Repentance.”
(Musnad Ahmad: 2/21)

(2)
It is possible that the Messenger of Allah (sallallahu alayhi wa sallam) used the very words mentioned in the hadith, as in another narration it is stated that he would supplicate with these words:
(Astaghfirullaha alladhi la ilaha illa huwa al-hayyul-qayyum wa atubu ilayh)
“I seek forgiveness from Allah, besides whom there is no deity worthy of worship. He is the Ever-Living, the Sustainer, and I turn to Him in repentance.”
(Jami‘ at-Tirmidhi, Tafsir al-Qur’an, Hadith: 3397)

(3)
The reasons for the Messenger of Allah (sallallahu alayhi wa sallam) seeking repentance and forgiveness were as follows:
➊ To express servitude (‘ubudiyyah).
➋ To teach the ummah.
➌ For humility and humbleness.
➍ He would seek forgiveness due to abandoning what was preferable (tark al-awla). Furthermore, in other ahadith it is clarified that the number of times he (sallallahu alayhi wa sallam) sought forgiveness would reach one hundred.
(Jami‘ at-Tirmidhi, Tafsir al-Qur’an, Hadith: 3259)

Explanation:
1:
So seek forgiveness for your sins and for the believing men and believing women. (: Muhammad: 19)