Reference : Hisn al-Muslim 164

اللَّهُمَّ اغْفِرْ لَهُ
اللَّهُمَّ ثَبِّتْهُ
Allāhumma’ghfir lah
Allāhumma thabbit’h.


O Allah, forgive him. 
O Allah, strengthen him.

Reference:
The Prophet (ﷺ) used to stop after burying the dead and say to the 
people: "Ask Allah to forgive your brother and pray for him to be 
strengthened, for indeed he is now being questioned." Abu Dawud 3/315, and 
Al-Hakim 1/370 who graded it authentic and Ath-Thahabi agreed.

Chapter: After burying the deceased