Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) dispatched a party towards Banu Lihyan and said: "Of every two men, one must go out for jihad, and the reward will be shared equally between them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17897