Imam al-Shafi'i (may Allah have mercy on him) said: It has been narrated from some people, as I say, that the Prophet (peace and blessings be upon him) "permitted them to eat the spoils in enemy territory, but (said) do not take anything of it out." If this is established from the Messenger of Allah (peace and blessings be upon him), then with it there is no other argument; and if it is not established because there is an unknown narrator in it, then likewise among the narrators from whom its permissibility is reported there is also an unknown narrator.
The Shaykh said: By the first, Imam al-Shafi'i meant the hadith of al-Waqidi, and by the second, what we mentioned after it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18007