"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."
Sunan Ibn Majah Book 12, Hadith 93
← Previous Hadith 'The Messenger of Allah (ﷺ) forbade selling food bought by measure until two Sa' have been measured - the Sa' of the seller and the Sa' of the buyer."

Sunan Ibn Majah Book 12, Hadith 92 Next Hadith → "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."'

Sunan Ibn Majah Book 12, Hadith 94