The Book of Transactions (Kitab Al-Buyu`)

Muslim :: Book 10 : Hadith 3873

Abu Sa'id (Allah be pleased with him) reported: We were given to eat, during the lifetime of Allah's Messenger (may peace be upon him), dates of different qualities mixed together, and we used to sell two sa's of these for one sa, (of fine quality of dates). This reached Allah's Messenger (may peace be upon him), whereupon he said: There should be no exchange of two sa's of (inferior) dates for one sa (of fine dates) and two sa's of (inferior) wheat for one sa' of (fine) wheat. and one dirham for two dirharms.

Source materials are from the University of Southern California MSA site
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