Reference : Hisn al-Muslim 159

اللهُـمِّ عَبْـدُكَ وَابْنُ أَمَـتِك،
احْتـاجَ إِلى رَحْمَـتِك،
وَأَنْتَ غَنِـيٌّ عَنْ عَذابِـه،
إِنْ كانَ مُحْـسِناً فَزِدْ في حَسَـناتِه،
وَإِنْ كانَ مُسـيئاً فَتَـجاوَزْ عَنْـه
Allāhumma `abduka wabnu 'amatik 
iḥtāja ilā raḥmatik, 
wa anta ghaniyyun `an `adhābih, 
in kāna muḥsinan fazid fī ḥasanātih, 
wa in kāna musī'an fatajāwaz `anh.


O Allah, Your male slave and the child of Your female slave is in need of Your mercy, 
and You are not in need of his torment. 
If he was pious then increase his rewards
and if he was a transgressor then pardon him.

Reference:
Al-Hakim 1/359 who graded it authentic and Ath-Thahabi agreed with him. See 
also Al-Albani, Ahkamul-Jana'iz, p. 125.

Chapter: For the deceased at the funeral prayer