Sufyan (may Allah have mercy on him) says that al-Hajjaj sent Musa bin al-Mughirah to collect zakah on vegetables. Musa bin Talhah (may Allah have mercy on him) said: We have the document of the Messenger of Allah (peace and blessings be upon him) which he (peace and blessings be upon him) gave to Mu'adh (may Allah be pleased with him); he (peace and blessings be upon him) commanded him: "That he should collect zakah only from wheat, barley, dates and raisins." Then he wrote this to al-Hajjaj, and he said: He is quite right.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7475