Musnad Ahmad 32:655

Khalaf ibn al-Walid narrated to us, he said: Abu Ja’far narrated to us, from al-Rabi’, from Anas ibn Malik, who said: The Messenger of Allah (may Allah bless him and grant him peace) forbade *al-nuhba* (plunder/looting). And he said: "Whoever plunders is not one of us."