We used to count that the Messenger of Allah ﷺ would say in a single sitting, "My Lord, forgive me and accept my repentance, for You are the Ever-Accepting of repentance, the All-Forgiving," a hundred times.
حدثنا ابن نمير عن مالك يعني ابن مغول عن محمد بن سوقة عن نافع
إن كنا لنعد لرسول الله ﷺ في المجلس يقول رب اغفر لي وتب علي إنك أنت التواب الغفور مائة مرة
Its isnad is sahih. Abu Dawud related it, 1:559-560, through Malik bin Mughul.Munziri said, 1460, Tirmidhi, Nasai, and Ibn Majah also related it. Tirmidhi said, hasan sahih ghareeb.In ح it reads "we were," and the correction is from ك.