Abd al-Malik bin Abi Hurrah narrates from his father that Sayyiduna Umar (may Allah be pleased with him) reserved for himself ten categories of al-Sawad: (1) the land of those killed in the fighting, (2) of those who fled to the disbelievers, (3) all the land of Kisra, (4) all the land that belonged to anyone of his (Kisra's) family, (5) every marshland where water collects, (6) every post-house. He said: I have forgotten four. Their total kharaj amounted to seven million. When the (battle of Dayr al-)Jamajim took place, the people burned the registers, and everyone took possession of whatever came into his hands.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18371