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". (ES)
English reference (ES): Book 2, Hadith 21 (ES)
Arabic reference (ES): Book 2, Hadith 22 (ES)