Shama-il Tirmidhi Chapter 002, Hadith Number 007 (021).

Abi Nadrah AI-'Awfee RA. reports: "I asked Abu Sa'eed Khudari Radiyallahu 'Anhu about the Seal of Prophethood of Rasulullah sallailahu 'Alayhi Wasallam. He said: 'It was a piece of raised flesh that was on Rasulullah Sallallahu 'Alayhi Wasallam's back".