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Hisn al-Muslim (Fortress of the Muslim) 195
((عَنِ ابْنِ عُمَرَ رضي الله عنه 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)).
(Ibn AAumar said: It would be counted that the Messenger of Allah would say one hundred times at any one sitting before getting up: (O my Lord, forgive me and turn towards me (to accept my repentance). Verily You are The Oft-Returning. The Oft-Forgiving.))
Cited in Hisn al-Muslim (Fortress of the Muslim) (No. 195).