"Dig a side niche in the grave for me, and set the bricks upright over me, just as was done for the Messenger of Allah ﷺ."
حدثنا ابن مهدي حدثنا عبد الله بن جعفر عن إسماعيل بن محمد عن أبيه عن سعد فذكر مثله ووافقه أبو سعيد على عامر بن سعد كما قال الخزاعي
Its isnad is sahih. In fact, it is really two isnads. Ahmad related it from Ibn Mahdi, and in his version, Ismail bin Muhammad bin Saad bin Abu Waqqas relates it from his father Muhammad, from his grandfather Saad. Then he indicated that Abu Saeed, the freedman of Banu Hashim, also related it, agreeing with Abu Salama Khuzai that Ismail bin Muhammad relates it from his uncle Amir bin Saad, as in the previous hadith. Both versions are sahih, from Muhammad bin Saad and from Amir bin Saad. Ibn Mahdi's version alone will come at 1489.