Musannaf Ibn Abi Shayba 13:1120

Abu Bakr narrated to us, he said: Isma'il ibn Ibrahim narrated to us, from Ayyub, from Nafi', who said: I was informed that Hakim ibn Hizam used to buy the documents of provisions (sakkak al-rizq). So, Umar forbade that one should sell them until he takes possession of them.