Allah Almighty says: "Take from their wealth a charity by which you purify them." [Surah al-Tawbah: 103]
And He said: "And give its due on the day of its harvest." [Surah al-An'am: 141]
And He said: "And from what We have brought forth for you from the earth." [Surah al-Baqarah: 267]
And the Prophet (peace and blessings be upon him) said: "There is no zakah on (produce) less than five wasqs."
And he (peace and blessings be upon him) said: "On (land) watered by the sky (rain) and springs, a tenth ('ushr) is due; and on what is watered by camels (or artificial means), a twentieth (half a tenth) is due."
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