Ibn 'Umar (May Allah be pleased with them) said:
We counted Messenger's saying a hundred times during one single sitting: Rabb- ighfir li, wa tubb 'alayya, innaka Antat-Tawwabur-Rahim. (My Rubb! Forgive me and pardon me. Indeed, You are the Oft-Returning with compassion and Ever Merciful."[Abu Dawud and At- Tirmidhi].
App reference: Book 20, Hadith 4
Arabic/English book reference: Book 20, Hadith 1872