It is narrated from Sayyiduna al-Harith bin Nawfal (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) taught them the supplication of the funeral prayer over the deceased: "O Allah, forgive our living and our dead, set right the affairs between us, and create affection between our hearts. O Allah, this is Your servant so-and-so, son of so-and-so; we know nothing of him but good, and You know him better than we do, so forgive us and him." Sayyiduna al-Harith (may Allah be pleased with him) said: I, being the youngest of the people, said to him (peace and blessings be upon him): What if I do not know good (about the deceased)? He (peace and blessings be upon him) said: "Then say only what you know."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3252