"We used to count that the Messenger of Allah (ﷺ) said one hundred times in a gathering: 'Rabbighfirli wa tub `alayya innaka Antat-Tawwabur-Rahim (O Allah forgive me and accept my repentance, for You are the Accepter of repentance, the Most Merciful).'"
Sunan Ibn Majah Book 33, Hadith 158