Kilayim, chapter 9, mishnah 8. This mishnah teaches us several of the general principles of the prohibition of kilayim, and how fibers must be treated in order for the transgression of kilayim to apply to them.
The words of the mishnah:
"Ein assur mishum kilayim ela tavui ve'arug" - there is no prohibition of kilayim except in fibers that have undergone two processes:
"Tavui" - the fibers mixed as kilayim must be spun into thread.
"Ve'arug" - they must be woven together, that is, joined to one another by weaving or by some other process of attachment. It is not enough for them simply to lie one on top of the other; an actual connection of some kind is required.
"Shene'emar: lo silbash sha'atnez":
The mishnah expounds the word "sha'atnez" and derives from it further details about what was stated at the beginning. "Sha'atnez" is a combination of three words:
"Shua" - the wool or the linen must be combed.
"Tavui" - afterwards the fibers must be spun.
"Nuz" - and finally they must be joined to one another by weaving or in some other manner.
These three words are hinted at in the word "sha'atnez": "shua," the letter tes of "tavui," and "nuz," which is the joining of the components together.
The dispute among the Rishonim:
The Rishonim disagree as to whether these processes need to be done to the wool by itself and to the linen by itself, or whether by Torah law they must be done to both of them together - combing them together, spinning them together and joining them together.
The exposition of Rabbi Shimon ben Elazar:
Rabbi Shimon ben Elazar takes a more homiletical approach to the second part of the word "sha'atnez" - "nuz." In his view it hints at the word "naloz", meaning crooked, "u'meliz hu es aviv shebashamayim alav" - he turns his Father in Heaven against him. That is, by committing this transgression he acts crookedly, and he causes the Holy One, as it were, to turn things against him because of the transgression of sha'atnez.
In summary: in this mishnah we learned that the prohibition of kilayim applies only to fibers that have been spun and joined together, and that it is not enough for them merely to lie one on top of the other. We saw the exposition of the word "sha'atnez" - shua, tavui and nuz, which are combing, spinning and joining; the dispute among the Rishonim as to whether these processes must be done to the wool and the linen together or to each one on its own; and the exposition of Rabbi Shimon ben Elazar on "nuz" from the word "naloz," that one who violates this prohibition turns his Father in Heaven against him.