We used to sit with Abu Saeed Khudri in Medina. I asked him about the seal of the Messenger of Allah ﷺ that was between his shoulders, and he gestured with his index finger like this, indicating a raised piece of flesh between the shoulders of the Messenger of Allah ﷺ.
حدثنا سريج حدثنا أبو ليلى قال أبي سماه سريج عبد الله بن ميسرة الخراساني
كنا نجالس أبا سعيد الخدري بالمدينة فسألته عن خاتم رسول الله ﷺ الذي كان بين كتفيه فقال بأصبعه السبابة هكذا لحم ناشز بين كتفيه ﷺ
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Its isnad is daif because of Abu Layla Abdullah bin Maysara. Ibn Main declared him daif, and others considered him unknown, and likewise Ghiyath Bakri, since he is in a better state than Abu Layla. Bukhari narrated the hadith with a different wording, 6/561, no. 2541, Fath al-Bari, in the Book of Virtues, chapter on the Seal of Prophethood, and Tirmidhi narrated it similarly, 5/602, no. 3644, and said, hasan sahih.