Yunus (may Allah have mercy on him) said: I asked al-Zuhri (may Allah have mercy on him) about the zakah of land on which there is a tax. He said: The Muslims always did so in the time of the Messenger of Allah (peace and blessings be upon him) and after him; they would work the land and rent it, and they would also pay zakah. We regard this land in the same way.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7498