Sahih Muslim Book 20, Hadith Number 4672.

Chapter : The merit of helping the warrior (fighting in the way of Allah) with something to ride upon and looking after his family in his absence.

It has been narrated (through a still different chain of transmitters) on the authority of Abu Sa’id Khudrl that the Messenger of Allah (may peace be upon him) despatched a force to Banu Lihyan. (and said:) One man from every two should join the force. Then he said to those who stayed behind: Those of you who will look well after the family and wealth of those who are going on the expedition will be getting half the reward of the warriors.