Ibn Umar (RA) said: Allah's Messenger (SAW) used to repeat in a single sitting:

رَبِّ اغْفِـرْ لي، وَتُبْ عَلَـيَّ، إِنَّكَ أَنْـتَ التَّـوّابُ الغَـفُورُ

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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.