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