Commentary for Sanhedrin 135:12

היכי עביד הכי והאנן תנן העושה מעשה חייב להתלמד שאני דאמר מר (דברים יח, ט) לא תלמד לעשות לעשות אי אתה למד אבל אתה למד להבין ולהורות:

Then they asked him, 'What of a shoe that is on the last?'<span class="x" onmousemove="('comment',' No utensil or garment could become unclean until it was quite ready for use. R. Eliezer and the Sages dispute with reference to a new shoe, ready for wear, but not yet removed from the last upon which it was made. The Rabbis maintained that it was a completely finished article, and hence liable to uncleanliness: whilst R. Eliezer held that until removed from its last it was not regarded as completely finished. ');">¹²</span>

Rashi on Sanhedrin