Abu Dawud Book 8, Hadith Number 2519.

Narated By AbuAyyub al-Ansari : AbuAyyub heard the Apostle of Allah (PBUH) say: Capitals will be conquered at your hands, and you will have to raise companies in large armies. A man will be unwilling to join a company, so he will escape from his people and go round the tribes offering himself to them, saying: Whose place may I take in such and such expedition? Whose place may I take in such and such expedition? Beware: That man is a hireling to the last drop of his blood.