Harithah bin Mudarrib narrates: Some people from Syria came to Sayyiduna Umar (may Allah be pleased with him) and said: "We have acquired some wealth, horses and slaves, and we would like zakah to be due from us on them, as a means of purification." Sayyiduna Umar (may Allah be pleased with him) said: "My two masters before me did not do this, that I should do it." Then he consulted the Companions of Sayyiduna Muhammad (peace and blessings be upon him), among whom was Sayyiduna Ali (may Allah be pleased with him). Sayyiduna Ali (may Allah be pleased with him) said: "It would be a good thing, provided it does not become a fixed jizyah that is formally imposed and collected after you as well."
Ahmad, and al-Tabarani in al-Mu'jam al-Kabir, recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الزكاة / 4372