It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "When you offer the funeral prayer, make your supplication for him with sincerity."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6964