Musannaf Ibn Abi Shayba 28:192

Al-Fadl ibn Dukayn narrated to us, he said: Muhammad ibn Talhah narrated to us, from Dawud ibn Sulayman, he said: Umar ibn Abd al-Aziz wrote to Abd al-Hamid ibn Abd al-Rahman: I command you to assess the land of their people—meaning the people of Kufa—and do not impose the burden of ruined land upon cultivated land, nor cultivated land upon ruined land. Look into the ruined land and take from it what it can bear, and improve it until it is cultivated. Do not take from the cultivated land anything except the prescribed land tax (kharaj), with gentleness and tranquility for the people of the land. I command you not to take for the land tax anything except the weight of seven (mithqals), and there shall be no "two" (extra) for it, nor the wages of the tax collectors, nor silver, nor the gift of Nayruz and Mihrajan, nor the price of the copy of the Quran, nor the wages of the conquests, nor the wages of the houses, nor the dirham of marriage, nor any land tax upon those who have accepted Islam from the people of the land.