The Messenger of Allah ﷺ said to me, "Abu Zayd, come close and rub my back." He uncovered his back, so I rubbed it, and the seal came between my fingers.He said, "I pressed it."Someone asked, "What is the seal?"He replied, "A cluster of hair on his shoulder."
قال لي رسول الله ﷺ: يا أبا زيد ادن مني وامسح ظهري. وكشف ظهره، فمسحت ظهره، وجعلت الخاتم بين أصابعي، قال: فغمزتها. قال: فقيل: وما الخاتم؟ قال: شعر مجتمع على كتفه.
Imam Ahmad (22889), Abu Yaala (6846) (and from him Ibn Hibban in his Sahih, 6300), and Tirmidhi in "Shamail" (19) all narrated it through Abu Asim Nabil, who said, Azra bin Thabit told us, Ilba bin Ahmar Yashkuri told us, Abu Zayd told us, and he related it. Its chain is sahih, and its narrators are the narrators of the Sahih. Abu Asim here is Dahhak bin Makhlad Nabil.Ahmad (20732) and Tabarani (17/27) also narrated it through two other routes from Azra bin Thabit, with a chain like it.Haythami said in "Majma" (8/281), "Ahmad, Abu Yaala, and Tabarani narrated it, and in one version with him it adds, I saw the seal on the back of the Messenger of Allah ﷺ like this on his back, as if it were an egg. One of its chains (I think he means the one from Ahmad) has narrators who are the narrators of the Sahih," meaning the first chain.