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."

حَدَّثَنَا سَهْلُ بْنُ أَبِي سَهْلٍ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ نُمَيْرٍ، عَنْ عُبَيْدِ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ كُنَّا نَشْتَرِي الطَّعَامَ مِنَ الرُّكْبَانِ جِزَافًا فَنَهَانَا رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ أَنْ نَبِيعَهُ حَتَّى نَنْقُلَهُ مِنْ مَكَانِهِ ‏.‏