Musnad Ahmad 1060:3

Yazid ibn Abd Rabbih narrated to us, Muhammad ibn Harb narrated to us, Abu Salamah narrated to me, from Yahya ibn Jabir, who said: I heard the nephew of Abu Ayyub al-Ansari mentioning, from Abu Ayyub, who said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "Indeed, the lands will be opened for you, and they will impose military expeditions upon you therein. A man among you will dislike the expedition, so he will separate himself from his people and offer himself to the tribes, saying: 'Who will suffice me against such-and-such an expedition?' Behold, that is the hired one until the last drop of his blood."