Reference for Bava Batra 187:13

אי הכי אמאי ימעט משום חומרא דכלאים

Every <i>se'ah</i> [of produce] which contains a quarter [of a <i>kab</i>] of another kind shall be reduced<span class="x" onmousemove="('comment',' To less than a quarter. ');">²⁴</span> [in order that it be permitted to be sown].<span class="x" onmousemove="('comment',' Two different kinds must not be sown together, in accordance with the prohibition of 'mingled seeds'. (Cf. Lev. XIX. 29.) ');">²⁵</span>

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