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عَنِ ابْنِ عُمَرَ رضي الله عنهما قَاَلَ: كَانَ يُعَدُّ لِرَسُولِ اللَّهِ صلى الله عليه وسلم فِي الْمَجْلِسِ الوَاحِدِ مِائَةُ مَرَّةٍ مِنْ قَبْلِ أَنْ يَقُومَ: (رَبِّ اغْفِرْ لِي, وَتُبْ عَلَيَّ, إِنَّكَ أَنْتَ التَّوَّابُ الغَفُورُ
It was narrated that Ibn 'Umar (may Allah be pleased with them both) said: 'In a single sitting, before he would rise, the Messenger of Allah (peace be upon him) would be counted as saying one hundred times: My Lord, forgive me and accept my repentance, for verily You are the Ever-Relenting, the All-Forgiving.'
Source: Sahih Ibn Majah 2/321, Sahih At-Tirmizi 3/153.