Amr bin Maymun bin Mihran (may Allah have mercy on him) said: I asked Umar bin Abd al-Aziz (may Allah have mercy on him) about the zakah of a Muslim who holds land subject to tax (kharaj): when zakah is asked of him, he says that the tax on it is also due from him. He said: The tax is on the land, and zakah is on the produce. He said: I asked him once more, and he gave the same answer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7497