We set out with Fadala bin Ubayd to the land of the Romans, where he was Muawiya's officer in charge of the mountain pass. A cousin of ours was killed, and Fadala performed the funeral prayer over him and stood by his grave until he was buried. When we had leveled the grave over him, Fadala said, "Make it inconspicuous, for the Messenger of Allah ﷺ used to command us to level the graves."
حدثنا محمد بن عبيد حدثنا محمد بن إسحاق
خرجنا مع فضالة بن عبيد إلى أرض الروم وكان عاملا لمعاوية على الدرب فأصيب ابن عم لنا فصلى عليه فضالة وقام على حفرته حتى واراه فلما سوينا عليه حفرته قال أخفوا عنه فإن رسول الله ﷺ كان يأمرنا بتسوية القبور
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Its isnad is sahih, its narrators are trustworthy and well known, and Ibn Ishaq is known for narrating from Thumamah. Muslim related the hadith in 2/666, no. 968. Abu Dawud in 3/215, no. 3219, Nasai in 4/88, no. 1030, and Bayhaqi in 3/411, all of them in the chapter on funerals. His saying "akhfu" means "khaffifu" (lighten), which is mentioned right after it.