I asked Abu Saeed Khudri about the seal of prophethood of the Messenger of Allah ﷺ. He said, "It was a raised piece of flesh on his back."
سألت أبا سعيد الخدري عن خاتم رسول الله ﷺ فقال: كان في ظهره بضعة ناشزة.
No references are available.
Tirmidhi reported it in Shamail (21) from Muhammad bin Bashshar, who said, Bishr bin Wadhah told us, Abu Aqeel Dawraqi told us, from Abu Nadra Awqi, and he related it.Its chain is hasan due to Bishr bin Wadhah, for he is hasan al-hadith. Abu Nadra Awqi, with a fatha on the unpointed letter and the waw, then a qaf, is Munzir bin Malik bin Qita, with a damma on the qaf and a fatha on the unpointed letter, and he is well known by his kunya, being among the men of the Sahih.Imam Ahmad reported it (11656) through another route with the wording, "flesh raised up between his shoulders." In it is Abdullah bin Maysara, who is daif. Haythami said in Majma (8/280), "Ahmad reported it, and in it is Abdullah bin Maysara, whom Ibn Hibban rated trustworthy while the majority rated him daif, and the rest of its men are trustworthy."I say, in it also is the teacher of Abdullah bin Maysara, Attab Bakri, whom no one rated trustworthy besides Ibn Hibban (5/274), and it was on this basis that Haythami relied in rating him trustworthy."Flesh raised up" means flesh elevated above the rest of the body.