Halacha requires that when one affixes the Sisit strings to the garment – whether to a Tallit Gadol (large Tallit) or a Tallit Katan (the small Tallit worn under one's shirt) – he must have specific intent to affix the strings for the purpose of the Misva of Sisit. If one affixed the Sisit strings without intent for the Misva, the Sisit are invalid and the garment may not be worn.
This requirement begins the moment one inserts the strings through the hole of the first corner of the garment. The Ben Ish Hai (Rabbi Yosef Haim of Baghdad, 1833-1909), in Parashat Lech-Lecha (3), writes that when inserting the Sisit strings through the hole of the garment one must verbally declare, "Hareni Matil Sisiyot Elu Le'shem Misvat Sisit" – "I hereby affix these strings for the purpose of the Misva of Sisit." It does not suffice to simply think in one's mind that he affixes the Sisit for the purpose of the Misva; he must make a verbal declaration to this effect.
If one did not make this verbal declaration when inserting the Sisit strings, but he did have intention for the Misva, are the Sisit valid, or must they be removed and inserted again with the required declaration?
The answer to this question may be inferred from an analysis of two rulings of the Shulhan Aruch regarding similar cases that arise with regard to other Misvot. When it comes to the writing of a Torah scroll, the Shulhan Aruch rules (Yoreh De'a 276:2) that when a person begins writing, and when he writes a Name of God in the scroll, he must verbally declare that he writes "Le'shem Kedushat Sefer Torah" ("for the purpose of the sanctity of the Torah scroll"). The implication of this ruling, as understood by later scholars, is that if one fails to make this verbal declaration the Sefer Torah is invalid for use. With respect to Tefillin, by contrast, the Shulhan Aruch writes (Orah Haim 38:8) that "it is proper" to verbally declare when tanning leather for the parchment that one does so for the purpose of the Misva of Tefillin. This formulation clearly suggests that this declaration is not indispensable, and that parchment produced without this declaration is nevertheless valid for use as Tefillin.
The commentaries explain that the Shulhan Aruch distinguishes between these two cases because there is some question as to whether tanning leather for Tefillin must be performed with specific intent at all. According to some authorities, Halacha does not require that the tanning process be done specifically with the intent for the Misva. Hence, although the Shulhan Aruch rules that such intent indeed is required, and that one must even verbally express this intent, he does not go so far as to disqualify parchment prepared without this verbal declaration. With regard to a Torah scroll, however, all authorities agree that writing without specific intent for the Misva disqualifies the scroll. The Shulhan Aruch therefore ruled stringently and disqualified a Torah scroll if the scribe did not verbally declare that his writing is performed for the sake of the Misva.
Applying this distinction to the case of Sisit leads us to the conclusion that a garment may, in fact, be worn even if no verbal declaration of intent was made when the Sisit strings were inserted into the holes on the corners. The Rambam (Rabbi Moshe Maimonides, 1135-1204) was of the opinion that no specific intent is required when one inserts the strings through the hole of the garment. Accordingly, despite the fact that Halacha does demand specific intent and even requires that one verbally declare this intent, the garment nevertheless remains valid should a person neglect to make this declaration, provided that he had the required intent in his mind. Given the controversy surrounding the issue of whether intent is required altogether, we may be lenient in a case where one intended for the Misva but did not verbally express that intent.
Summary: One who affixes Sisit to a Tallit must have in mind that he does so specifically for the purpose of the Misva of Sisit, and he must verbally express this intention by declaring, "Hareni Matil Sisiyot Elu Le'shem Misvat Sisit." This declaration should be made when one inserts the Sisit strings through the holes on the corners of the garment. If one did not make this declaration, the Sisit are nevertheless valid, provided that he at least thought in his mind that he affixes the Sisit for the purpose of the Misva.
