It wasnarrated that Abu Hurairah said:
"I heard the Messenger ofAllah(ﷺ) say: 'When you offer the prayer for the deceased,supplicatesincerely for him.'" (FR)
English reference (FR): Vol. 1, Book 6, Hadith 1497 (FR)
Arabic reference (FR): Book 6, Hadith 1564 (FR)