Musnad Abu Ya'la al-Mawsili 65:15

Khalf ibn Hisham narrated to us, Abu 'Awanah narrated to us, from al-Aswad, from Jundub ibn Sufyan al-Bajali: That the Messenger of Allah (may Allah bless him and grant him peace) had his finger bleed during one of the battles. He said: "You are nothing but a finger that has bled, and in the path of Allah is what you have encountered."