Reference for Bava Metzia 179:5

אלא לא קשיא הא במעשר ראשון הא במעשר שני ואיבעית אימא הא והא במעשר שני ולא קשיא הא רבי מאיר הא רבי יהודה

but if an Israelite threshes with a heathen's beast, he does. Thus the rulings on <i>terumah</i> are contradictory, and likewise those on tithes. Now, as for the rulings on <i>terumah</i>, it is well, and there is no difficulty: the one refers to <i>terumah</i> [itself]; the other to the produce of <i>terumah</i>;<span class="x" onmousemove="('comment',' With respect to the former there is no prohibitions as explained on p. 516, n. 7. But if it were sown and produced a further crop, Biblically speaking it is not terumah at all, but ordinary hullin, though by a Rabbinical enactment it ranks as such. Since the Rabbis cannot nullify a Scriptural prohibition, the injunction, Thou shalt not muzzle, remains in force. The reason for this Rabbinical measure was that otherwise the Israelite might evade his obligations by separating terumah and then resowing it. Also, should a priest possess defiled terumah, which may not be eaten, he might keep it for resowing, when likewise it reverts to hullin by Scriptural law; but whilst keeping it he might forget its defiled nature and eat it. ');">⁵</span>

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