Abu Dawud Book 34, Hadith Number 4516.

Chapter : Should a muslim be killed for an infidel.

Narated By Abdullah ibn Amr ibn al-‘As : The Apostle of Allah (PBUH) said, mentioning the tradition similar to the one transmitted by Ali. This version adds: The most distant of them gives protection as from all, those who are strong among them send back (spoil) to those who are weak among them, and their expeditions sending it back to those who are at home.