He reported God’s Messenger as saying, "Do not sell gold for gold unless it is like for like, and do not make one amount greater than the other; do not sell silver for silver unless it is like for like, and do not make one amount greater than the other; and do not sell for ready money something to be given later*.” A version has, "Do not sell gold for gold or silver for silver unless both are of equal weight.”
(Bukhari and Muslim.)
* Payment is not to be made till the goods are received.
Abu Sa'id al-Khudri reported God’s Messenger as saying, "Gold is to be paid for by gold, silver by silver, wheat by wheat, barley by barley, dates by dates, and salt by salt, like for like, payment being made on the spot. If anyone gives more or asks more he has dealt in usury. The receiver and the giver are equally guilty.”
‘Ubada b. as-Samit reported God's Messenger as saying, "Gold is to be paid for by gold, silver by silver, wheat by wheat, barley by barley, dates by dates, and salt by salt, like for like and equal for equal, payment being made on the spot. If these classes differ, sell as you wish if payment is made on the spot.”