Sahih Muslim Book 19, Hadith Number 4346.

Chapter : Fai’ (property taken from the enemy without a formal war).

It has been narrated on the authority of Abu Huraira that the Messenger of Allah (may peace be upon him) said: If you come to a township (which has surrendered without a formal war) and stay therein, you have a share (that will be in the form of an award) in (the properties obtained from) it. If a township disobeys Allah and His Messenger (and actually fights against the Muslims) one-fifth of the booty seized there-from is for Allah and His Apostle and the rest is for you.