Reference for Bava Metzia 122:13
א"ר אבהו עד כאן של תורה מכאן ואילך של דבריהם וכן אמר רבא עד כאן של תורה מכאן ואילך של דבריהם
AND WHAT IS <i>TARBITH</i>? THE TAKING OF INTEREST ON PRODUCE. E.G., IF ONE PURCHASES WHEAT AT A GOLD <i>DENAR</i>, etc. Is then the preceding example<span class="x" onmousemove="('comment',' [Lit., 'is that stated' according to MS.M.; cur. edd. 'Are all these stated'.] Viz., lending a sela' that five denarii should be returned. ');"><sup>25</sup></span> not interest? — R. Abbahu said: Hitherto it [i.e., the first instance] is interest in the Biblical sense, but from here onward by Rabbinical law.<span class="x" onmousemove="('comment',' Lit., 'according to these words'. Lending a sum of money for a larger return is Biblically forbidden; but buying ahead, as illustrated in the Mishnah, was prohibited by the Rabbis. ');"><sup>26</sup></span>