Harithah bin Mudarrib said that people from Sham came to Umar (may Allah be pleased with him) and said: "We have acquired a great deal of wealth, horses and slaves, and we would like there to be zakah and purification for us in it." He said: "My two companions (Muhammad (peace and blessings be upon him) and Abu Bakr al-Siddiq (may Allah be pleased with him)) did not do this, so how can I do it!" Then Umar (may Allah be pleased with him) consulted Ali (may Allah be pleased with him) in the presence of a group of the Companions of the Messenger of Allah (peace and blessings be upon him), and Ali (may Allah be pleased with him) said: "It is good, provided it does not become a jizyah (tax) that is levied on them on a fixed, regular basis."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7414