Sayyiduna Abu Musa and Sayyiduna Mu'adh bin Jabal (may Allah be pleased with them both) relate that the Messenger of Allah (peace and blessings be upon him) sent the two of them to Yemen and commanded them to teach the people the rulings of their religion, and said: "Do not take zakah from anything except these four kinds; the four kinds are: barley, wheat, raisins and dates."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7451