Ibn 'Umar (radhi-yAllaahu 'anhumaa) said: It would be counted that at any one sitting, the Messenger of Allaah (sal-Allaahu 'alayhe wa sallam) before getting up would say one hundred times:
رَبِّ اغْفِرْ لِي وَتُبْ عَلَىَّ إِنَّكَ أَنْتَ التَّوَّابُ الْغَفُورُ
My Lord, forgive me and accept my repentance, You are the Ever-Relenting, the All-Forgiving.
Rabbighfir lee wa tub 'alayya 'innaka 'Antat-Tawwaabul-Ghafoor.
Sahih Ibn Majah 2/321. See also Al-Albani, Sahih At-Tirmithi 3/153.