Musannaf Ibn Abi Shayba 36:329

Abu Bakr narrated to us, he said: Abd al-Rahim ibn Sulayman narrated to us, from Shayban, from Yahya ibn Abi Kathir, from Sa'id ibn Abi Sa'id, the freed slave of al-Mahri, that Abu Sa'id informed him that the Messenger of Allah (may Allah bless him and grant him peace) said to them during a military expedition he undertook against Banu Lihyan: "Let one man from every two men set forth, and the reward is between them."