From Sahih Muslim (Hadith 4574):
It has been narrated on the authority of Abu Huraira that the Messenger of Allah (ﷺ) 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 therefrom is for Allah and His Apostle and the rest is for you
Reference: Muslim 4574