Du'a for the deceased in need of Allah's mercy

اللَّهُمَّ عَبْدُكَ وَابْنُ أَمَتِكَ، احْتَاجَ إِلَى رَحْمَتِكَ، وَأَنْتَ غَنِيٌّ عَنْ عَذَابِهِ، إِنْ كَانَ مُحْسِنًا فَزِدْ فِي حَسَنَاتِهِ، وَإِنْ كَانَ مُسِيئًا فَتَجَاوَزْ عَنْهُ

Allahumma 'abduka wabnu amatika, ihtaja ila rahmatika, wa anta ghaniyyun 'an 'adhabihi, in kana muhsinan fazid fi hasanatihi, wa in kana musi'an fatajawaz 'anhu.

O Allah, Your servant and the son of Your maidservant is in need of Your Mercy and You are without need of his punishment. If he was righteous then increase his reward and if he did wrong, overlook his wrongdoing.

Muwaṭṭaʾ Mālik 536