Musannaf Ibn Abi Shayba 36:338

Abu Usama narrated to us, from Abd al-Rahman ibn Yazid ibn Jabir, from Makhul, who said: Al-Hajjaj ibn Abd Allah al-Nasri narrated to me, saying: The *nafl* (spoils of war) is the right of the *nafl* of the Messenger of Allah, may Allah bless him and grant him peace.