Sahih Muslim Book 19, Hadith Number 4377.

Chapter : Justification for taking food in the land of the enemy.

It has been narrated on the authority of Abdullah b. Mughaffal who said I found a bag containing fat on the day of the Battle of Khaibar. I caught hold of it and said: I will not give anything today from it to anybody. Then I turned round and saw that the Messenger of Allah (may peace be upon him) was smiling (at my words).