Musannaf Ibn Abi Shayba 28:787

'Abd al-Rahman ibn Mahdi narrated to us, from Muhammad ibn Rashid, from Makhul, who said: The khums (one-fifth) is in the status of the fay' (spoils taken without fighting); the Imam gives from it to the rich and the poor. He said: And Layth ibn Abi Ruqayya informed me that 'Umar ibn 'Abd al-'Aziz wrote that the path of the khums is the path of the general fay'.