Musannaf Ibn Abi Shayba 28:753

Sharīk narrated to us, from Jābir, from al-Qāsim, from his father, who said: ‘Abdullāh said: "The *nafl* (spoils of war granted before the division) is [permissible] as long as the two ranks or the two armies have not met. But when the two ranks or the two armies have met, then it is *maghnam* (general booty to be divided)."