Musannaf Ibn Abi Shayba 13:1372

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Ayyub, from Nafi’, who said: It was narrated to me that Hakim ibn Hizam used to buy the documents of provisions (sakkak al-rizq). Umar forbade him from selling them until he had taken possession of them.