"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."  

كُنَّا نَشْتَرِي الطَّعَامَ مِنَ الرُّكْبَانِ جِزَافًا فَنَهَانَا رَسُولُ اللَّهِ ﷺ أَنْ نَبِيعَهُ حَتَّى نَنْقُلَهُ مِنْ مَكَانِهِ  

"I used to sell dates in the marketplace, and I would say: 'This was such and such an amount (when I bought it).' I Would give the purchaser a specific amount of dates according to the way it had been measured for me, and take my profit. Then I began to have some doubts about that, so I asked the Messenger of Allah ﷺ, and he said: ‘When you name the amount, measure it in front of the purchaser."'  

كُنْتُ أَبِيعُ التَّمْرَ فِي السُّوقِ فَأَقُولُ كِلْتُ فِي وَسْقِي هَذَا كَذَا فَأَدْفَعُ أَوْسَاقَ التَّمْرِ بِكَيْلِهِ وَآخُذُ شِفِّي فَدَخَلَنِي مِنْ ذَلِكَ شَىْءٌ فَسَأَلْتُ رَسُولَ اللَّهِ ﷺ فَقَالَ إِذَا سَمَّيْتَ الْكَيْلَ فَكِلْهُ