We used to count for the Messenger of Allah ﷺ in a single sitting one hundred times that he said, "My Lord, forgive me and accept my repentance. Indeed, You are the Acceptor of repentance, the Merciful."
إن كنا لنعد الرسول الله ﷺ في المجلس الواحد مائة مرة: رب اغفر لي، وتب علي، إنك أنت التواب الرحيم.
Recorded by Abu Dawud (1516), Tirmidhi (3434), Ibn Majah (3814), Ahmad (4726), Bukhari in Adab al-Mufrad (618), Nasai in Amal al-Yawm wal-Layla (458), and Ibn Sunni (371), and Ibn Hibban graded it sahih (927), all of them through the route of Muhammad bin Suqa, from Nafi, from Ibn Umar, who related it. Its chain is sahih. Tirmidhi said, "This is a hasan sahih gharib hadith."