He asked Saad bin Abu Waqqas about exchanging white wheat for unhusked barley. Saad asked him, "Which of the two is better?"He replied, "The white wheat."So Saad forbade him from doing that and said, "I heard the Messenger of Allah ﷺ being asked about buying dried dates with fresh dates. The Messenger of Allah ﷺ asked, 'Do fresh dates decrease in weight when they dry out?' They said, 'Yes.' So the Messenger of Allah ﷺ forbade that."
حدثنا عبد الله بن مسلمة عن مالك عن عبد الله بن يزيد
أنه سأل سعد بن أبي وقاص عن البيضاء بالسلت فقال له سعد أيهما أفضل قال البيضاء فنهاه عن ذلك وقال سمعت رسول الله ﷺ يسأل عن شراء التمر بالرطب فقال رسول الله ﷺ أينقص الرطب إذا يبس قالوا نعم فنهاه رسول الله ﷺ عن ذلك
Sourcing, its isnad is hasan. Tirmidhi narrated it in the chapter on business transactions, section on what was said regarding the prohibition of muhaqalah and muzabanah, hadith no. 1225, and Nasai, hadith no. 4549, and Ibn Majah, hadith no. 2264, from the hadith of Malik through this route, and it is found in Muwatta (Yahya), 2/624.Tirmidhi said, hasan sahih, and Ibn Jarud graded it sahih, hadith no. 657, as did Hakim, 2/38-39, and Dhahabi concurred with him.