Bulugh al-Maram, Book 3, Hadith 35 (In-Book)

Abu Hurairah (RAA) narrated that the Messenger of Allah said: “If you offer the funeral prayer for a deceased person, supplicate Allah sincerely for him." Related by Abu Dawud.

وَعَنْهُ أَنَّ اَلنَّبِيَّ ‏- صلى الله عليه وسلم ‏-قَالَ: { إِذَا صَلَّيْتُمْ عَلَى اَلْمَيِّتِ فَأَخْلِصُوا لَهُ اَلدُّعَاءَ } رَوَاهُ أَبُو دَاوُدَ, وَصَحَّحَهُ ابْنُ حِبَّانَ 1‏.‏ ‏1 ‏- حسن.‏ رواه أبو داود (3199)‏، وابن حبان (3076)‏.‏

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also referenced under: Book 3, Hadith 591 (English)