وَقَالَ صلى الله عليه وسلم: ((إِنَّهُ لَيُغَانُ عَلَى قَلْبِي وَإِنِّي لأَسْتَغْفِرُ اللَّهَ فِي الْيَوْمِ مِائَةَ مَرَّةٍ)).
(He also said: ‘verily my heart becomes preoccupied, and verily I seek Allah’s forgiveness a hundred times a day.)(preoccupied: i.e. in a state of ‘forgetfulness’. The Prophet always used to increase in his remembrance of his Lord, in attaining a nearness to Allah and having consciousness of Allah to the extent that if this intensity lessened in anyway, he would regard it as a sin and would then race to seek forgiveness from Allah.)
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.