عَنِ ابْنِ عُمَرَ قَالَ : كَانَ يُعَدُّ لِرَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي الْمَجْلِسِ الْوَاحِدِ مِائَةَ مَرَّةٍ مِنْ قَبْلِ أَنْ يَقُومَ "رَبِّ اغْفِرْ لِي وَتُبْ عَلَيَّ إِنَّكَ أَنْتَ التَّوَّابُ الْغَفُورُ"
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.
Sahih Ibn Majah 2/321. See also Al-Albani, Sahih At-Tirmidhi 3/153.