Abu Dawud Book 8, Hadith Number 2698.

Narated By Abdullah ibn AbuAwfa : Muhammad ibn AbulMujahid reported Abdullah ibn AbuAwfa as saying: I asked: Did you set aside the fifth of the food in the time of the Apostle of Allah (PBUH)? He replied: On the day of Khaybar we captured food and a man would come and take as much food of it as needed and then go away.