I heard the Messenger of Allah ﷺ say, "When you pray over the deceased, make your supplication for him sincere."
سمعت رسول الله ﷺ يقول: إذا صليتم على الميت فأخلصوا له الدعاء.
Abu Dawud (3199) and Ibn Majah (1497) both narrated it through Muhammad bin Salama Harrani, from Muhammad bin Ishaq, from Muhammad bin Ibrahim, from Abu Salama bin Abdul-Rahman, from Abu Hurairah, and he related it.Its chain is hasan because of Muhammad bin Ishaq, who was a mudallis, so when he states his source explicitly, his hadith is judged hasan. This explicit statement of direct narration occurs in Ibn Hibban (3077), where he narrated it through another route from Muhammad bin Ishaq, who said, Muhammad bin Ibrahim narrated to me, from Saeed bin Musayab, Abu Salama bin Abdul-Rahman, and Salman Aghar, the freed slave of Juhayna, all of whom narrated to me from Abu Hurairah, and he related the hadith.