Gemara

90b (Mishnah): If one was Matzni'a (stored away) for planting, a sample or for a cure, and was Motzi on Shabbos, he is liable for any amount;

Question: Why does it say that he was Matzni'a? It should say that one who was Motzi for planting, a sample or a cure is liable for any amount!

Answer (Abaye): The case is, he was Matzni'a and later forgot why he was Matzni'a, and was Motzi Stam (without intent):

One might have thought that he nullified his original intent. The Mishnah teaches that this is not so. One always acts according to his original intent.

Menachos 42a - Version #1 (Rav Yehudah citing Rav): "Bnei Yisrael v'Asu Lahem Tzitzis" teaches that Yisraelim can make Tzitzis, but Nochrim cannot!

42b - Version #2 (Rav Yehudah citing Rav): A Nochri can make Tzitzis. "Bnei Yisrael v'Asu Lahem Tzitzis" teaches that others may make Tzitzis for Yisrael. (end of Version #2)

(Rav Yehudah citing Rav): If Tzitzis were made from Kotzim, Nimim or Gardin (these will be explained), they are Pasul.

If they were made from Sisin (a certain plant), they are Kosher.

(Shmuel): Even from Sisin, they are Pasul. Tzitzis must be spun Lishmah.

Rishonim

Rambam (Hilchos Tzitzis 1:12): If a Nochri made Tzitzis, it is Pasul, for it says "Daber El Bnei Yisrael v'Asu Lahem Tzitzis." If a Yisrael made Tzitzis without intent, it is Kosher. If Tzitzis were made from what was already made, it is Pasul.

Rosh (Hilchos Tzitzis 14): Rav Yehudah taught that if Tzitzis were made from Kotzim, Nimim or Gardin, it is Pasul. Kotzim, Kotzim are strings that hang from loom threads and protrude like thorns. It is normal to cut them off. Nimim are strings taken from a garment to be sewn. Gardin are strings that come out from the end of a garment. It is Pasul because they were not attached Lishmah.

Poskim

Shulchan Aruch (OC 14:2): If a Yisrael inserted Tzitzis in a garment without intent, if no other Tzitzis are [found] to be Machshir it, one may rely on the Rambam, who is Machshir, but he does not bless on it.

Beis Yosef (DH Kasav): The Rambam is Machshir if a Yisrael made Tzitzis without intent. He must refer to inserting the strings and making the windings and knots. Surely, the strings must be spun l'Shem Tzitzis, like it says in Menachos 42b. He holds that if Tzitzis must be also made (inserted, wound and tied) Lishmah, why do we teach that Tzitzis that a Nochri made? Also if a Yisrael made them Lo Lishmah, they are Pesulim! Also, why do we need the Drashah "Bnei Yisrael v'Asu", and not Nochrim? The same source that requires spinning Lishmah should disqualify Nochrim! Rather, it is Pasul only through a Nochri [but not through a Yisrael Lo Lishmah]. However, in Sukah 9a, Rashi and the Rosh explain that Rav Yehudah disqualifies Tzitzis made from Kotzim, Nimim or Gardin because they were not attached Lishmah. Perhaps also the first knot, which is required mid'Oraisa, must be done l'Shem Tzitzis. The Rambam holds that the Pesul from Kotzim, Nimim or Gardin is not due to Lishmah.

Beis Yosef (DH u'Mihu): I question the Rambam's source to be Machshir a Yisrael who made Tzitzis without intent. Do not say that if we required Kavanah Lishmah, we would not need to disqualify a Nochri, for even a Yisrael without intent is Pasul. Rav does not require spinning Lishmah. This shows that he does not require Lishmah at all. He is Machshir from Sisin. Therefore, he disqualifies only if a Nochri made them. We disqualify from Sisin, for we require spinning Lishmah. We can say that the entire making of Tzitzis must be Lishmah! Perhaps the Rambam holds that if so, when Rav Yehudah told Shmuel Rav's opinion, Shmuel should have said that even if a Yisrael made them Lo Lishmah it is Pasul. Rather, Shmuel requires only spinning Lishmah. Also, if Shmuel required attaching Lishmah, Rav should have taught that if a Yisrael made them Lo Lishmah it is Kosher, to teach unlike Shmuel. Also, it seems that all need to expound "Bnei Yisrael v'Asu" to exclude Nochrim. This implies that if a Yisrael made them Lo Lishmah, it is Kosher, for Nochrim cannot make them Lishmah. L'Halachah, one should be concerned for Rashi and the Rosh and make them with intent. If they were made without intent, and no other Tzitzis are found to fix it, one may rely on the Rambam.

Taz (2): "Bnei Yisrael v'Asu" refers to inserting the strings, for it is said regarding the corners [in which the strings are inserted]. If Shmuel required Lishmah, the Gemara should have asked what he learns from the verse.

Magen Avraham (4): Why doesn't he remove them from the garment and reattach them Lishmah? We must say that we discuss on Shabbos. Alternatively, one of the ends snapped. If he unties them, it is forbidden to put them on the garment. [Only remnants less than the full Shi'ur are Kosher.]

Mishnah Berurah (7): We can say that he has no time to remove them and reattach them Lishmah, for it is close to Shabbos.

Machatzis ha'Shekel: Perhaps also the Magen Avraham means that it is close to Shabbos, for the Mechaber connotes that if he had other strings, he would not allow to rely on the Rambam. Alternatively, the Mechaber means that he is not able to attach other strings.

Kaf ha'Chayim (10): Alternatively, the Tzitzis are very thin and worn, so if he unties them, they are like to snap.

Gra (DH Hitil): The Rambam is consistent with his opinion that the Pesul of Gardin is not due to Lishmah. (Rather, it is disgraceful.) A proof is from Rav, who [in Version #1] is Machshir Tzitzis of a Nochri but disqualifies a Get, due to Lishmah (Gitin 23a). Even Version #2 disqualifies only due to a Drashah "Bnei Yisrael", but not due to Lishmah. In Sukah we say only that spinning must be Lishmah. In Menachos, Rashi disqualifies from Gardin because it was already made, but not due to Lishmah. See below 15:4. Tosfos and the Rosh disqualify Gardin due to Lishmah. There is no proof from Get, like it says in Avodah Zarah 27a, that R. Yehudah holds that a Nochri is Kosher for Milah. See what I wrote in 11:2,5.

Damesek Eliezer: Tosfos holds that there is no proof from Get, because the entire Toref (the information particular to this couple) must be Lishmah. Tzitzis and Milah require only that the beginning be Lishmah.

Mishnah Berurah (6): This refers to making the first knot, which is mid'Oraisa, without intent. If the first knot was with intent, even though the other knots and windings were without intent, it is Kosher even if one has other Tzitzis.

Mishnah Berurah (8): Even if only the insertion was Lo Lishmah, he may not bless on it, and all the more so if the first knot was Lo Lishmah. Rashi, the Rosh and Tosfos require the verse to disqualify a Nochri even if a Yisrael supervises and teaches him to make it Lishmah. Therefore, l'Chatchilah, before inserting in the garment, he should explicitly say that he inserts all the strings l'Shem Tzitzis.

Kaf ha'Chayim (13): Likewise, one may not wear them in Reshus ha'Rabim [on Shabbos].

Bi'ur Halachah (DH Hitil): The Beis Yosef says that the Rambam is Machshir even if the Yisrael explicitly said that he inserts Lo Lishmah. Rashi and the Rosh disqualify even if he did so Stam. If not, the Rosh could agree with the Rambam! Surely, from Kotzim and Nimim were not inserted Lishmah. Also the Drishah (1) says so. See R. Akiva Eiger (Teshuvah 4), who answers why we don't say that Stam it is destined to be done Lishmah. Mishkenos Yakov (20) holds that the Rambam is lenient only Stam, for it is destined to be done Lishmah.

Bi'ur Halachah (DH Ba'al ha'Itur): If he mentally intended Lishmah, I say that he may bless on them. Regarding spinning, I did not resolve this. Here, it is permitted. Firstly, it is a Sefek-Sefeka (two doubts). Perhaps the Halachah is that intent suffices, and perhaps the Halachah follows the Rambam. Also, Stam threads are not destined to be [spun] for Tzitzis, but Stam threads are inserted [in a four cornered garment] l'Shem Tzitzis. Through joining these reasons, one may bless on it. However, l'Chatchilah one should explicitly say that he does it Lishmah. Also, even if he intended only while attaching the first string, and later he attached Stam, he can bless, for we say that everyone acts according to his original intent, even though seemingly, they are distinct acts. A proof is from Tosfos (Zevachim 2b DH Ha). If one intended Lishmah in Shechitah and Kabalah was Stam, even if we say that Stam Kabalah is not destined to be Lishmah, one acts according to his original intent. I said so above (11:2) regarding spinning and twining. Do not say that there [is different, for], all four Avodos are one Mitzvah. Also here, all four [corners with] Tzitziyos are one Mitzvah, and they are Me'akev each other! If he says at the beginning "all the Tzitzis I will attach, all are l'Shem Tzitzis", we do not need this discussion.

Kaf ha'Chayim (8): The Mordechai brought two opinions about whether Stam is like Lishmah. Therefore, one should be careful to explicitly say that it is Lishmah.

Bi'ur Halachah (DH Lo): If one forgot to intend while inserting, and intended when making the knot, perhaps he may bless, for the end proves about the beginning. This does not apply to Kotzim. Also, the Rambam does not require Lishmah, and perhaps Stam is Lishmah. However, some Poskim (Maharshal and the Drishah) never say that the end proves about the beginning. This requires investigation. I think that the argument is only when one Yisrael inserted for another, like the case of a Nochri inserting. Then, perhaps the Yisrael inserts merely to do his friend's request. (If he explicitly asked him to insert them Lishmah, all agree that it is Kosher.) If one inserts for himself, perhaps the Rosh agrees that Stam is Lishmah, especially for our Talesim which are purely for the Mitzvah.