Abu Hurairah (May Allah be pleased with him) reported:
I heard the Messenger of Allah (ﷺ) saying: "When you pray over the dead, make a sincere supplication for him."[Abu Dawud and Ibn Majah]. (ID)
App reference (ID): Book 7, Hadith 43 (ID)
Arabic/English book reference (ID): Book 7, Hadith 937 (ID)