Reference : Hisn al-Muslim 248

قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم: "وَاللَّهِ إِنِّي لأَسْتَغفِرُ اللَّهَ وَأَتُوبُ إِلَيْهِ فِي الْيَوْمِ أَكْثَرَ مِنْ سَبْعِينَ مَرَّةٍ"
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Allah's Messenger (ﷺ) said: "By Allah, I seek the forgiveness of Allah, and repent to Him more than seventy times in a day.

Reference:
Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 11/101.

Chapter: Seeking forgiveness and repentance