Musnad Ahmad 1240:4

Hassan narrated to us, he said: Ibn Lahi'ah narrated to us, he said: Abu al-Nadr narrated to us, that Busr ibn Sa'id narrated to him, from Ma'mar ibn Abdullah, that he sent a servant of his with a sa' of wheat and said to him: "Sell it, then buy barley with it." The servant went and took a sa' and an addition of some of a sa'. When he came to Ma'mar, he informed him of that. Ma'mar said to him: "Did you do that? Go and return it, and do not take anything except like for like, for I used to hear the Messenger of Allah, peace and blessings of Allah be upon him, saying: 'Food for food, like for like.'" And our food on that day was barley. It was said: "But it is not the same." He said: "I fear that it may resemble [usury]."