Ibn Umar (Radi Allahu anhu) said Allah’s Messenger (Sallal laahoo alaihi wassallam) used to repeat in a single sitting
رَبِّ اغْفِـرْ لي، وَتُبْ عَلَـيَّ، إِنَّكَ أَنْـتَ التَّـوّابُ الغَـفُورُ
“My Lord, forgive me and accept my repentance, You are the Ever-Relenting, the All-Forgiving.”
Rabbighfir lee wa tub ‘alayya ‘innaka ‘Antat-Tawwaabul-Ghafoor.
Sahih Ibn Majah 2/321. See also Al-Albani, Sahih At-Tirmidhi 3/153.