It was narrated that Abu Laila Al-Kindi said:
"Khabbab came to `Umar and said: 'Come close, for no one deserves this meeting more than you, except `Ammar.' Then Khabbab started to show him the marks on his back where the idolaters had tortured him."
English reference: Vol. 1, Book 1, Hadith 153
Arabic reference: Book 1, Hadith 158