Ibn Umar (RA) said: Allah's Messenger(SAW) used to repeat in a single sitting:
195. Rabbighfir lee wa tub 'alayya 'innaka 'Antat-Tawwaabul-Ghafoor.
My Lord, forgive me and accept my repentance, You are the Ever-Relenting, the All-Forgiving.
Reference: Sahih Ibn Majah 2/321. See also Al-Albani, Sahih At-Tirmithi 3/153.