Reference for Menachot 75:9

דתניא מצוה להקדים לבן לתכלת ואם הקדים תכלת ללבן יצא אלא שחיסר מצוה מאי חיסר מצוה

But the Rabbis [say].' That ye may look upon it', signifies each one by itself. Must we then say that [our Mishnah] is not in accordance with Rabbi? - Rab Judah answered in the name of Rab, You may even say that it follows Rabbi's view, for [our Mishnah deals here] only with the question of precedence. As it was taught: The [proper performance of the] precept is to insert<span class="x" onmousemove="('comment',' Or, to twine, v. Sh. Mek. n. 3. The white threads, as derived above from 'the corner', precede the blue in the verse.');">⁸</span> the white threads before the blue; but if a man inserted the blue before the white, it is indeed valid, but he has not fulfilled the precept. What is meant by 'has not fulfilled the precept'?

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