Reference : Hisn al-Muslim 253

.وَقَالَ صلى الله عليه وسلم: إِنَّهُ لَيُغَانُ عَلَى قَلْبِي وَإِنِّي لأَسْتَغْفِرُ اللَّهَ فِي الْيَوْمِ مِائَةَ مَرَّةٍ.
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Allah's Messenger (ﷺ) said: "It is a heavy thing for my heart if I do not seek Allah's forgiveness a hundred times a day."

Reference:
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.

Chapter: Seeking forgiveness and repentance