Allah's Messenger ( صلی اللہ علیہ وسلم ) used to repeat in a single sitting: Rabbigh’fir lī wa tub `alayy innaka antat-Tawwābu ‘l-Ghafūr.
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Ibn Umar (ra) said: Allah's Messenger ( صلی اللہ علیہ وسلم ) used to repeat in a single sitting:

My Lord, forgive me, and accept my repentance, You are the Ever-Relenting, the All-Forgiving.