Nazir 58a-b

Although the Torah forbids a man from removing the hair around his head - pe'at roshkhem - and from shaving his beard - pe'at zekanekhah (see Vayikra 19:27), nevertheless, in the case of a nazir who has completed his nezirut and must remove all of his hair, he is permitted to do so. The underlying principle that allows him to do this is the rule that aseh doheh lo ta'aseh - fulfillment of a positive commandment "pushes aside" a negative commandment.

One of the sources offered by the Gemara for the idea of aseh doheh lo ta'aseh is the commandment of tzitzit. Although the Torah forbids sha'atnez (a mixture of wool and linen fibers - see Devarim 22:11), immediately following we find the commandment to place gedilim (tzitzit) on one's clothing (see verse 12). This is understood by the Sages to permit sha'atnez when placing tzitzit on a garment.

The Shittah Mekubetzet offers two explanations for this case. One is simply that wool tzitzit are placed on a linen garment, which is permitted for the mitzvah even though the two fibers will be connected. The second suggestion is that the strand of tekhelet - the blue strand - which is always made of wool, can be combined with strands of linen when tying the tzitzit.

Another lesson that the Gemara suggests may be derived from this pasuk (=verse) is that ordinarily tzitzit should be made from the same material as the garment. The exceptions are wool and linen, which can be used to make tzitzit no matter what the garment is made of. Rava derives this from the fact that the Torah teaches that tzitzit are placed on the corners of the garment (see Bamidbar 15:38) which is understood to mean that the tzitzit should match the corner - i.e. they should be made from the same material - yet the passage in Devarim that places the prohibition of sha'atnez and the commandment of tzitzit in the same context implies that tzitzit should be made of either linen or wool.