Shama-il Tirmidhi Chapter 002, Hadith Number 008 (022).

Abdullah bin Sarjas Radiyallahu 'Anhu says: "I came to Rasulullah Sallallahu 'Alayhi Wasallam while there were people sitting in his company. I went around to the back of Rasulullah Sallallahu 'Alayhi Wasallam (The narrator may have done this physically). Rasulullah Sallallahu 'Alayhi Wasallam understood what I was trying to do. He removed the sheet (body wrap) from his back. I saw the place of the Seal of Prophethood between his two shoulders. It was like a cluster surrounded by til (moles) which appeared to be like a wart. I came before Rasulullah Sallallahu 'Alayhi Wasallam and said to him. May Allah forgive you (or Allah has forgiven you, as is mentioned in Surah Fath, 'That Allah may forgive you, your former and your latter sins'). Rasulullah Sallallahu 'Alayhi Wasallam replied: 'May Allah forgive you too'. The people said to me that Rasulullah Sallallahu'Alayhi Wasallam has made du'aa for your maghfirah (forgiveness). I replied: 'Yes, and for you too, because Allah Ta'aala has said: 'O Muhammad, seek forgiveness for yourself and the Mu'min males and females also"'. (That is why Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam asked for the forgiveness of Muslims).