Musannaf Ibn Abi Shayba 28:771

'Isa ibn Yunus narrated to us, from Salih ibn Abi al-Akhdar, from al-Walid ibn Hisham, from Malik ibn 'Abd Allah al-Khath'ami, who said: We were sitting with 'Uthman, and he said, "Who is here from the people of al-Sham?" So I stood up. He said, "Convey to Mu'awiyah that when he acquires spoils of war, he should take five shares, then write upon one of them 'for Allah,' then he should cast lots; so whichever one it comes out upon, he should take it."