Musnad Ahmad 31:124

Hassan narrated to us, Ibn Lahi'ah narrated to us, Ibn Hubayrah narrated to us, from Hanash ibn Abdullah, from Abu Sa'id al-Khudri—my father said: It is not *marfu'* (attributed to the Prophet)—he said: "A forward sale (*salam*) is not valid for wheat, barley, and *sult* (a type of barley) until they are threshed, nor for grapes, olives, and the like until they are pressed. Nor [is it valid to exchange] gold in hand for silver on credit, nor silver on credit for gold in hand."