Maliks Muwatta Book 17, Hadith Number 32.

Yahya related to me from Malik that Zayd ibn Aslam said, ”Umar ibn al Khattab drank some milk which he liked (very much) and he asked the man who had given it to him, ‘Where did this milk come from?’ The man told him that he had come to a watering-place, which he named, and had found grazing livestock from the zakat watering there. He was given some of their milk, which he then put into his water-skin, and that was the milk in question. Umar ibn al-Khattab then put his hand into his mouth to make himself vomit.”

Malik said, “The position with us is that if anyone refuses to honour one of the obligatory demands of Allah, and the muslims are unable to get it, then they have the right to fight him until they get it from him.”