סימן ט סעיף יז
יֵשׁ אוֹמְרִים שֶׁבְּכָל הַשֶּׁטַח שֶׁל הַכָּנָף שֶׁרָאוּי לְהַטִּיל אֶת הַצִּיצָה, דְּהַיְנוּ מִמְּלֹא קֶשֶׁר גּוּדָל מִשְּׂפַת הַטַּלִּית עַד שְׁלשָׁה גוּדָלִין, לֹא יִהְיֶה שָׁם שׁוּם תֶּפֶר אֲפִלּוּ מַשֶּׁהוּ בְּחוּט שֶׁהָיָה רָאוּי לַעֲשׂוֹת מֵחוּטִין כָּאֵלּוּ צִיצִית לְטַלִּית זֶה, כְּגוֹן אִם הַטַּלִּית שֶׁל פִּשְׁתָּן, לֹא יִתְפֹּר שָׁם בְּחוּטֵי פִֹּשְתָּן רַק בְּחוּטֵי מֶשִׁי וְכַדּוֹמֶה. וְאִם הַטַּלִּית שֶׁל מֶשִׁי, לֹא יִתְפְּרֶנּוּ בְּחוּטֵי מֶשִׁי. וְאִם הַטַּלִּית שֶׁל צֶמֶר, לֹא יִתְפְּרֶנּוּ בְּחוּטֵי צֶמֶר, אֶלָּא בְּשֶׁל מֶשִׁי וְכַדּוֹמֶה. וְיֵשׁ לְהַחְמִיר בָּזֶה גַּם בָּאִמְרָא שֶׁעוֹשִׂין סָבִיב הַנֶּקֶב לְחַזְּקוֹ. וְכָל זֹאת אֵינוֹ רַק בְּחוּט לָבָן. אֲבָל בְּחוּט צָבוּעַ, אֵין לָחוּשׁ (סִימָן ט"ו).
Rashi's opinion(Maseches Menachos 41a) is that any thread used in the stitching at the corner of the garment should be of material other than the tzitzis. This is because there is concern that one may leave some of the sewing thread dangling from the corner and then combine it with three other strings for tzitzis. Since his original intent for this thread was for stitching and not for tzitzis, it cannot become part of the tzitzis merely by his decision to consider it so. This constitutes Ta'aseh velo min he'asui.
Some authorities hold: that on the entire area of the corner, which is eligible for attaching the tzitzis, that is, from after the length of the first thumb knuckle, from the edge of the tallis until a length of three thumb-breadths,47See glossary. there should not be any stitches whatsoever with a thread that may be used in the making of tzitzis for this particular tallis;48See preface for the reason. for example, if the tallis, was made of linen, one should not sew there with linen threads, but [use] only silk threads or the like.49I.e., or any thread not valid for tzitzis in this tallis. [Likewise] if the tallis is silk it should not be sewn with silk threads. If the tallis was made of wool it should not be sewn with woolen threads but with silken threads or the like. It is preferable to adhere to this strict view also in regard to the hem that is made around the hole to reinforce it (the hole). All of the above, however, applies only to white thread but with colored thread50Which is not to be used for tzitzis.there is no reason for concern.51And the thread used at the “corner” may be of the same material as the tallis.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)