Ibn Umar (ra) said: Allah's Messenger (ﷺ) used to repeat in a single sitting: Rabbigh’fir lī wa tub `alayy innaka antat-Tawwābu ‘l-Ghafūr. Ibn Umar (ra) said: Allah's Messenger (ﷺ) used to repeat in a single sitting: 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-Tirmidhi 3/153.