The Rambam codifies the laws of tzitzis in detail, discussing how strings are to be prepared and the conditions for valid tzitzis. He does not mention avoiding scissors specifically, making his silence relevant to later debates about whether this is normative law or merely custom.
חוּטֵי הַצִּיצִית בֵּין לָבָן בֵּין תְּכֵלֶת צְרִיכִין טְוִיָּה לְשֵׁם צִיצִית. וְאֵין עוֹשִׂין אוֹתָן לֹא מִן הַצֶּמֶר הַנֶּאֱחָז בַּקּוֹצִים כְּשֶׁהַצֹּאן רוֹבְצִין בֵּינֵיהֶם. וְלֹא מִן הַנִּימִין הַנִּתְלָשִׁין מִן הַבְּהֵמָה. וְלֹא מִשִּׁיּוּרֵי שְׁתִי שֶׁהָאוֹרֵג מְשַׁיֵּר בְּסוֹף הַבֶּגֶד. אֶלָּא מִן הַגִּזָּה שֶׁל צֶמֶר אוֹ מִן הַפִּשְׁתָּן. וְאֵין עוֹשִׂין אוֹתָן מִצֶּמֶר הַגָּזוּל וְלֹא מִשֶּׁל עִיר הַנִּדַּחַת וְלֹא מִשֶּׁל קָדָשִׁים. וְאִם עָשָׂה פָּסוּל. הַמִּשְׁתַּחֲוֶה לִבְהֵמָה צַמְרָהּ פָּסוּל לְצִיצִית. אֲבָל הַמִּשְׁתַּחֲוֶה לְפִשְׁתָּן הַנָּטוּעַ הֲרֵי זֶה כָּשֵׁר שֶׁהֲרֵי נִשְׁתַּנָּה:
Both the white strands of the tzitzit and those dyedtechelet must be spun for the sake of being used for [the mitzvah of] tzitzit. [Tzitzit] may not be made from wool which becomes attached to thorns when sheep graze among them, nor from hairs which are pulled off the animal, and not from the leftover strands of the woof which the weaver leaves over when he completes a garment. Rather, they must be made from shorn wool or from flax. [Tzitzit] may not be made from wool which was stolen, which came from an ir hanidachat, or which came from a consecrated animal. If such wool was used, it is unacceptable. If a person bows down to an animal, its wool is not acceptable for use for tzitzit. If, however, one bows down to flax which is planted, it is acceptable, because it has been changed.