It is narrated from Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever has his head wounded in the path of Allah and hopes for the reward, Allah forgives him all his previous sins."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13650