Abdullah bin al-Harith (may Allah have mercy on him) narrates from his father, Sayyiduna al-Harith (may Allah be pleased with him), that the Prophet (peace and blessings be upon him) taught them the funeral prayer (and taught them to recite this supplication in it): 'Allahumma-ghfir li-ahya'ina wa amwatina, wa aslih dhata baynina, wa allif bayna qulubina. Allahumma hadha abduka fulanu bnu fulanin la na'lamu illa khayran, wa anta a'lamu bihi, fa-ghfir lana wa lahu' (O Allah, forgive our living and our dead, set right our mutual relations 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, who was the youngest of the group, said to him (peace and blessings be upon him): "And if I do not know (any) good (about him)?" He (peace and blessings be upon him) said: "Then say only what you know."
No one narrated this hadith from Alqamah bin Marthad except Layth bin Abi Sulaym; Hammam is alone in narrating it, and this hadith is narrated from al-Harith (may Allah be pleased with him) only with this chain.
Hadith Referenceالمعجم الأوسط / باب الميم / 5913
Hadith Takhrij«أخرجه الطحاوي فى «شرح مشكل الآثار» برقم: 975، والطبراني فى «الكبير» برقم: 3265»