Musnad Ahmad 35:18

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from the like of it, he said: "As long as they have not separated." Rawh narrated to us, Ibn Jurayj narrated to us, ‘Ata’ informed me that Safwan ibn Muwahhib informed him, from ‘Abdullah ibn Muhammad ibn Sayfi, from Hakim ibn Hizam, he said: The Messenger of Allah (may Allah bless him and grant him peace) said to me: "Has it not come to me—or has it not reached me, or as Allah willed of that—that you sell food?" He said: "Yes, O Messenger of Allah." The Messenger of Allah (may Allah bless him and grant him peace) said: "Do not sell food until you have purchased it and taken full possession of it." ‘Ata’ said: And ‘Abdullah ibn ‘Ismah al-Jushami also informed me of it, that he heard Hakim ibn Hizam narrating it to him, from the Prophet (may Allah bless him and grant him peace).