Abu Sa'id Al-Khudri (May Allah be pleased with him) reported

The Messenger of Allah (ﷺ) sent an expedition to Banu Lahyan and said, "One man from every two should join the fighting force, and the reward will be shared amongst them equally."

[Muslim].

Another narration in Muslim is: The Messenger of Allah (ﷺ) said, "Let one of every two men go forth", and added: "Whoever stays behind (and looks well after the family and the property of those who have joined the expedition) will get half the reward of the warrior."