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.
رَبِّ اغْفِرْ لِي وَتُبْ عَلَيَّ إنَّكَ أَنْتَ التَّوَّابُ الرَّحِيمُ
My Rubb! Forgive me and pardon me. Indeed, You are the Oft-Returning with compassion and Ever Merciful.”
[Abu Dawud and At-Tirmidhi]
Authentic Dua & Dhikr 