It is narrated from Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) that dates were brought to the Messenger of Allah (peace and blessings be upon him). He (peace be upon him) said: "Are these not from among our dates?" Upon this, a man said: "O Messenger of Allah (peace be upon him), we gave two sa' of our own dates in exchange for one sa' of these." He (peace be upon him) said: "This is usury. Return it, then sell your own dates for a price, and then with that price, buy these dates separately for us."
Hadith Referenceالسنن المأثورة / باب في البيوع / 219
Hadith Takhrijصحیح مسلم ، البيوع، باب بيع الطعام مثلاً بمثل رقم : 1594 ۔