Dua #1114•Muslim 4/2075. Ibn 'Athir explains that the Prophet (ﷺ) was always vigilant in his remembrance and drawing near to Allah, and if he forgot to do any of what he normally did from time to time, or it slipped his mind, he felt as if he had wronged himself and so he would begin to seek the forgiveness of Allah. See Jami'ul-'Usool 4/386.

Seeking forgiveness and repentance

Hisnul Muslim
#1114

.وَقَالَ صلى الله عليه وسلم: إِنَّهُ لَيُغَانُ عَلَى قَلْبِي وَإِنِّي لأَسْتَغْفِرُ اللَّهَ فِي الْيَوْمِ مِائَةَ مَرَّةٍ.

Translation

Al-Agharr al-Muzani, who was one amongst the Companions (of the Holy Prophet) reported that Allah's Messenger (ﷺ) said:There is (at times) some sort of shade upon my heart, and I seek forgiveness from Allah a hundred times a day