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."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / Q7497
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
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
Alqamah (may Allah have mercy on him) narrates from Abdullah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Tax and a tenth are not both combined upon a Muslim."
This hadith is false (batil) as a connected and marfu' (raised to the Prophet) report, and Yahya bin 'Anbasah is accused of fabrication. Ibn Adi (may Allah have mercy on him) said: Abu Hanifah narrates it only from Hammad from Ibrahim as Ibrahim's own saying; Yahya bin 'Anbasah narrated it from Abu Hanifah and connected it to the Prophet (peace and blessings be upon him). Yahya bin 'Anbasah's weakness is evident, because he narrates fabricated reports from trustworthy narrators.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7499