It is narrated from Sayyiduna Zaid bin Aslam (may Allah be pleased with him) that Sayyiduna Umar bin Khattab (may Allah be pleased with him) drank some milk and found it pleasant. He asked, "Where did this milk come from?" The one who had brought it replied, "I went to a watering place," and he mentioned its name, "where the animals of zakat were drinking water. The people there milked them and gave it to me. I put it in my water-skin, and this is the same milk that you drank." So Sayyiduna Umar (may Allah be pleased with him) put his hand in his mouth and vomited.

Imam Malik, may Allah have mercy on him, said: In our view, the ruling is that whoever withholds that which Allah has ordained and does not allow the Muslims to take it, then it is obligatory upon the Muslims to fight against that person until they take that right.