Reference : Hisn al-Muslim 158

اللهُـمِّ إِنَّ فُلانَ بْنَ فُلانٍ في ذِمَّـتِك ، 
وَحَبْـلِ جِـوارِك ، 
فَقِـهِ مِنْ فِتْـنَةِ الْقَـبْرِ وَعَذابِ النّـار ، 
وَأَنْتَ أَهْلُ الْوَفـاءِ وَالْـحَقِّ ، 
فَاغْفِـرْ لَهُ وَارْحَمْـهُ ، 
إِنَّكَ أَنْتَ الغَـفورُ الـرَّحيم
Allāhumma inna [name the person] fī dhimmatik, 
wa ḥabli jiwārik, 
faqihi min fitnati ‘l-qabri wa `adhābin-nār, 
wa anta ahlu ‘l-wafā'i wa ‘l-ḥaqq. 
Faghfir lahu warḥamh
innaka anta ‘l-Ghafūrur-Raḥīm.


O Allah, surely [name the person] is under Your protection,
and in the rope of Your security,
so save him from the trial of the grave and from the punishment of the Fire.
You fulfill promises and grant rights,
so forgive him and have mercy on him.
Surely You are Most Forgiving, Most Merciful.

Reference:
Ibn Majah, Abu Dawud 3/211. See also Al Albani, Sahih Ibn Majah 1/251.

Chapter: For the deceased at the funeral prayer