Sahih al-Bukhari 2123, 2124

Ibrahim ibn al-Mundhir narrated to us: Abu Damra narrated to us: Musa narrated to us, from Nafi, who said: Ibn Umar told us that they used to buy food from the caravans in the time of the Prophet. He would send someone to prevent them from selling it at the place where they had bought it, until they transported it to the place where food was sold. He also said: Ibn Umar, may Allah be pleased with them both, narrated to us that the Prophet forbade selling food after buying it until one had taken full possession of it.