The Book of Jihad and Expedition (Kitab Al-Jihad wa'l-Siyar)
Muslim :: Book 19 : Hadith 4426
It has been narrated on the authority of Jundub b. Sufyan who said: A finger of the Messenger of Allah (may peace be upon him) was wounded in one of the encounters He said: Thou art just a little finger which has bled, and what thou hast experienced is in the cause of Allah.