It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Sell gold for gold only like for like, and do not make one exceed the other. Likewise, take silver for silver only like for like, and do not make one exceed the other; and do not sell what is in hand for what is deferred." In the narration of Abu Nasr it is: "Do not sell what is in hand for what is deferred."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10475