Musannaf Ibn Abi Shayba 28:508

Waki’ narrated to us, he said: Abdullah bin al-Walid al-Muzani narrated to us, he said: A man informed me—whose father was the most knowledgeable of people regarding this Sawad (the fertile lands of Iraq), and he is called Abd al-Malik bin Abi Hurra—from his father, that Umar bin al-Khattab selected ten lands from the lands of the Sawad. He said: I counted seven and forgot three: the marshes (al-ajam), the water-sinking areas (maghid al-ma’), the land of Chosroes, the Monastery of the Post (Dayr al-Barid), the land of those who were killed in the battle, and the land of those who fled. He said: It remained in the register (diwan) like that until al-Hajjaj burned the register, so every group took what was adjacent to them.