Musnad Ahmad 27:1731

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Nafi' narrated to me, from Ibn 'Umar, who said: Tell them, "That the Messenger of Allah, peace and blessings of Allah be upon him, used to send someone to them when they bought food from the caravans, to prevent them from selling it until they had brought it to their own dwellings."