Sahih Muslim
The Book of Jihad and Expedition (Kitab Al-Jihad wal-Siyar)
Book Number: 019 / Hadith Number: 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.