Sunan Ibn Majah Book 33, Hadith 158
"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).'"
"The Messenger of Allah (ﷺ) said to me: 'You should recite Subhan-Allah, wal-Hamdu-Lillah, wa la ilahah illallah, wa Allahu Akbar (Glory is to Allah, praise is to Allah, none has the right to be worshiped but Allah, and Allah is the Most Great), for it sheds sins like the tree sheds its leaves.'"
the Messenger of Allah (saas) said: 'I seek the forgiveness of Allah and repent to Him one hundred times each day.'