Praise be to Allah.
There is no reliable hadith that has been narrated concerning the wording of the prayer for forgiveness and supplication for the deceased after the burial, as far as we know. Rather the instruction is narrated in general terms to seek forgiveness for him and to offer supplication, asking that he be made steadfast [when questioned in the grave by the angels]. So in order to comply with this instruction, it is sufficient to say any words asking for forgiveness and offering supplication for him, such as saying: “O Allah, forgive him and make him steadfast in adhering to the truth,” and so on.
And Allah is the source of strength. May Allah send blessings and peace upon our Prophet Muhammad and his family and companions. End quote.
Shaykh ‘Abd al-‘Aziz ibn Baz, Shaykh ‘Abd ar-Razzaaq ‘Afifi, Shaykh ‘Abdullah ibn Qa‘ud.
Fatawa al-Lajnah ad-Da’imah li’l-Ifta’ (9/94).
And Allah knows best.