Musannaf Ibn Abi Shayba 28:188

Waki’ narrated to us, he said: Ibn Abi Layla narrated to us, from al-Hakam, that ‘Umar ibn al-Khattab sent ‘Uthman ibn Hunayf to as-Sawad. He imposed upon every jarib—whether cultivated or uncultivated, and whether reached by water or not—one dirham and one qafiz, meaning wheat and barley. And upon a jarib of vineyards, [he imposed] ten [dirhams], and upon a jarib of fresh produce, five [dirhams].