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]. (JA)
App reference (JA): Book 7, Hadith 43 (JA)
Arabic/English book reference (JA): Book 7, Hadith 937 (JA)