Musnad Ahmad 27:948

Hassan narrated to us, Ibn Lahi'ah narrated to us, Ubayd Allah ibn Abi Ja'far narrated to us, from Nafi', from Ibn Umar, who said: "I saw the spoils of war being divided into five parts, then lots were cast for them. Whatever was for the Messenger of Allah, may Allah bless him and grant him peace, was his, and he would choose [from it]."