Musar for Shabbat 166:1
כרעו הן טהורין כמאן אזלא הא דתניא כל הטמאות המסיטות טהורות חוץ מהיסטו של זב שלא מצינו לו חבר בכל התורה כולה לימא דלא כרבי עקיבא דאי כרבי עקיבא איכא נמי ע"ז אפילו תימא ר' עקיבא תנא זב וכל דדמי ליה
if they out weigh [him], they are clean.<span class="x" onmousemove="('comment',' For they bear the zab, and only articles which are fit for lying or sitting upon, or human beings, are unclean in such a case. ');"><sup>1</sup></span> With whom does that which was taught agree, [viz.,]: [As for] all unclean things which move [others], they [the things moved] are clean, save [in the case of] moving by a <i>zab</i>, for which no analogy<span class="x" onmousemove="('comment',' Lit., 'companion'. ');"><sup>2</sup></span> is found in the whole Torah. Shall we say that this is not according to R. Akiba, for if according to R. Akiba, there is an idol too? — You may even say that it agrees with R. Akiba: He states <i>zab</i> and all that is like thereto.<span class="x" onmousemove="('comment',' Which includes an idol, since R. Akiba deduces an idol's power to contaminate from a niddah, who is akin to a zab. ');"><sup>3</sup></span>