Sahih Bukhari Volume 005, Book 057, Hadith Number 067.

Narated By 'Urwa : On the day of the battle of Al-Yarmuk, the companions of the Prophet said to Az-Zubair, "Will you attack the enemy vigorously so that we may attack them along with you?" So Az-Zubair attacked them, and they inflicted two wounds over his shoulder, and in between these two wounds there was an old scar he had received on the day of the battle of Badr When I was a child, I used to insert my fingers into those scars in play.