Reference : Hisn al-Muslim 96

أَسْتَغْفِرُ اللَّهَ وَأَتُوبُ إِلَيْهِ 
(مِائَةَ مَرَّةٍ فِي الْيَوْمِ)
Astaghfirullāha wa atūbu ilayh.

I seek the forgiveness of Allah and repent to Him. (Recite one hundred times in Arabic during the day.)

Reference:
Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 11/101, Muslim 4/2075.

Chapter: In the morning and evening