It was narrated that Ibn 'Umar said:
"We used to buy food from troops of riders (i.e., the caravans) without knowing the amount, but the Messenger of Allah (ﷺ) forbade us to sell it until it had been delivered to us."
English reference: Vol. 3, Book 12, Hadith 2229
Arabic reference: Book 12, Hadith 2314