Sunan Ibn Majah, Chapter: The Chapters on Business Transactions

"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 (saw), and he said: 'When you name the amount, measure it in front of the purchaser."'