Sunan Ibn Majah 2230
It was narrated that 'Uthman bin 'Affan said:
"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."'
حَدَّثَنَا عَلِيُّ بْنُ مَيْمُونٍ الرَّقِّيُّ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يَزِيدَ، عَنِ ابْنِ لَهِيعَةَ، عَنْ مُوسَى بْنِ وَرْدَانَ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، عَنْ عُثْمَانَ بْنِ عَفَّانَ، قَالَ كُنْتُ أَبِيعُ التَّمْرَ فِي السُّوقِ فَأَقُولُ كِلْتُ فِي وَسْقِي هَذَا كَذَا ‏.‏ فَأَدْفَعُ أَوْسَاقَ التَّمْرِ بِكَيْلِهِ وَآخُذُ شِفِّي فَدَخَلَنِي مِنْ ذَلِكَ شَىْءٌ فَسَأَلْتُ رَسُولَ اللَّهِ ـ صلى الله عليه وسلم ـ فَقَالَ ‏
"‏ إِذَا سَمَّيْتَ الْكَيْلَ فَكِلْهُ ‏"‏ ‏.‏