From Sahih Bukhari (Hadith 7351):
Narrated Abu Sa`id Al-Khudri and Abu Huraira:Allah's Messenger (ﷺ) sent the brother of the tribe of Bani Adi Al-Ansari as governor of Khaibar. Then the man returned, bringing Janib (a good kind of date). Allah's Messenger (ﷺ) asked him, "Are all the dates of Khaibar like that?" He replied, "No, by Allah, O Allah's Messenger (ﷺ)! We take one Sa' of these (good) dates for two Sas of mixed dates." Allah's Messenger (ﷺ) then said, "Do not do so. You should either take one Sa of this (kind) for one Sa' of the other; or sell one kind and then buy with its price the other kind (of dates), and you should do the same in weighing
Reference: Bukhari 7351