Reference : Hisn al-Muslim 263

كَانَ الرَّجُلُ إِذَا أَسْلَمَ عَلَّمَهُ النَّبيُّ صلى الله عليه وسلم الصَّلاَةَ ثُمَّ أَمَرَهُ أَنْ يَدْعُوَ بِهَؤُلاَءِ الْكَلِمَاتِ: (اللَّهُمَّ اغْفِرِ لِي، وَارْحَمْنِي، وَاهْدِنِي، وَعَافِنِي وَارْزُقْنِي)
Whenever anyone accepted Islam, the Prophet (ﷺ) used to teach him how to 
pray then he would instruct him to invoke Allah with the following words:
Allāhummaghfir lī, warḥamnī, waḥdinī, wa `āfinī warzuqnī.

Whenever anyone accepted Islam, the Prophet (ﷺ) used to teach him how to pray then he would instruct him to invoke Allah with the following words:

O Allah forgive me, and have mercy on me and guide me and give me good health and provide for me.

Reference:
Muslim 4/2073, and in one of Muslim's reports there is the addition: 'For 
these words combine [the goodness of] this world and the next.'

Chapter: Excellence of Tasbih, Tahmid, Tahlil, and Takbir