Sahih Muslim Book 20, Hadith Number 4670.

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 on the authority of Abu Sa’id Khudri that the Messenger of Allah (may peace be upon him) sent a force to Banu Lihyan (who are from Banu Hudhail, and said: One man from every two and the reward (will be divided) between the two.