Gemara

Nazir 20b (Mishnah): If Reuven said 'I am a Nazir'; another (heard him and) said 'and I'; (and another said) 'and I', they are all Nezirim. If Reuven permitted his Nezirus, the Nezirus of all of them is permitted. If the last permitted his Nezirus, he is not a Nazir, but the rest of them are Nezirim.

(Reish Lakish): This is when each said 'and I' Toch Kedei Dibur.

Toch Kedei Dibur is the time needed to greet someone with Shalom, i.e. the time needed for a Talmid to greet his Rebbi (Shalom Alecha Rebbi).

Objection (R. Yehudah Nesi'ah): This does not leave time for a Talmid (who wanted to accept Nezirus like Reuven, and his Rebbi came. He must greet his Rebbi, and then it is too late to say 'and I'!)

Support (for Reish Lakish - Beraisa): If Reuven said 'I am a Nazir', Shimon paused Kedei Dibur, and said 'and I', Reuven is a Nazir, but Shimon is not.

Kedei Dibur is the time for a Talmid to greet his Rebbi.

20.a (Mishnah): If Reuven said 'I am a Nazir', and Shimon said 'my mouth is like his mouth', or 'my hair is like his hair', Shimon is also a Nazir.

21a - Contradiction (Beraisa): If one said 'my hand is a Nezirah'; or, 'my foot is a Nezirah', these words do nothing;

If he said 'my head is a Nezirah' or 'my liver is a Nezirah', he is a Nazir;

The rule is, if he accepted Nezirus on a part of the body that is vital to life, he is a Nazir.

Answer (Rav Yehudah): He means 'my mouth is like his mouth regarding wine' or 'my hair is like his hair regarding cutting.'

Nedarim 14a: We learn from "Ish Ki Yidor Neder" - Hatfasah must be in Davar ha'Nadur, not in Davar ha'Asur (something the Torah forbids).

Shevuos 20a (Beraisa): If one said 'eating meat and drinking wine are forbidden to me like the day that Gedalyah ben Achikam was killed', he is forbidden.

(Shmuel): The case is, he had already taken a Neder never to eat meat or drink wine on that day.

One might have thought, since he is forbidden to eat that day (Tzom Gedalyah) even without his Neder, it is as if he was Matfis in a forbidden matter, not in Davar ha'Nadur. The Beraisa teaches that this is not so.

Rishonim

The Rif and Rosh (Nedarim 3b and 2:1) bring the Gemara on 14a.

Rosh: "Ki Yidor Neder" teaches that Hatfasah must be in Davar ha'Nadur, i.e. something that became Kadosh through speech. Hatfasah in a vow is like vowing (Nazir 20b). One can forbid meat to himself like he did on a previous day.

Rif and Rosh (Shevuos 7b and 3:1): Hatfasah in a Neder takes effect. A Mishnah (Nazir 22a) teaches that if one said 'I am a Nazir' and another heard and said 'and I', all of them are Nezirim.

Question: The Mishnah is only when the others said 'and I' Toch Kedei Dibur. Hatfasah works even after Kedei Dibur!

Answer (Ran 8a): If they said 'I am like him', it works even after Kedei Dibur. Since they said only 'and I', if this was after Kedei Dibur, it is meaningless.

Question: Perhaps it is not Hatfasah. Rather, it is as if they said 'I am a Nazir'!

Answer (Ran): The Seifa teaches that if the first permitted his vow, all of them are permitted. This shows that they meant 'I am like him.'

Rambam (Hilchos Nezirus 2:4): If Reuven said 'I am a Nazir', and Levi said 'my mouth is like his mouth regarding wine' or 'my hair is like his hair regarding cutting', he is a Nazir. Similarly, if he heard and said 'and I' Toch Kedei Dibur, and Shimon heard and said 'and I' Toch Kedei Dibur of Levi, even if there are 100, they are all Nezirim.

Rambam (Hilchos Nedarim 3:3): If Levi heard Reuven vow and said 'and I am like you' Toch Kedei Dibur, he is forbidden what is forbidden to Reuven. If Shimon heard Levi and said 'and I', even if there are 100, and each said 'and I' Toch Kedei Dibur of the other, they are all forbidden.

Mishneh l'Melech: When he says 'and I', he refers to what the previous one said, so it must be Toch Kedei Dibur. But when he says 'I am like you', why must it be Toch Kedei Dibur? In Halachah 4, the Rambam does not require Toch Kedei Dibur for Hatfasah of a food in another food! If one sees a Nazir and says 'I am like him', he becomes a Nazir (even if he did not hear his acceptance)! The Ran (above) and Tosfos Yom Tov (Nazir 4:1) say so.

Rambam (4): Similarly, if Reuven said 'this meat is forbidden to me', and even many days later 'and this bread is like this meat', the bread is Nitpas and is forbidden. If he said later 'and this honey is like this bread, and this wine is like this honey', even if there are 100, they are all forbidden.

Rosh (Shevuos 3:3): Our texts say '...because it (Tzom Gedalya) is forbidden only mid'Rabanan.' When he vows it becomes Asur mid'Oraisa, and when he is Matfis in it, "Lo Yachel" applies to the new matter. Rashi rejects this text, for even if the Isur is mid'Oraisa, Nedarim take effect on Mitzvos. It seems that this is no reason to reject it. A Neder takes effect on an Isur mid'Oraisa and one transgresses "Lo Yachel." However, if he was Matfis something else in it, it is not forbidden, for he is Matfis in the initial Isur, so this is not Davar ha'Nadur. If the initial Isur was mid'Rabanan, it is considered Davar ha'Nadur.

Poskim

Shulchan Aruch (204:1): If one says 'this loaf is forbidden like a Korban' or 'it will be forbidden to me' or 'it is Isar to me', it is forbidden. The same applies if he forbade a loaf and made it like Davar ha'Nadur, and said 'this is like this.' Even if there are 100, all are forbidden. Or, if he vowed to fast or not to eat meat one day, and he said that another day should be like that day, it is forbidden.

Beis Yosef (DH u'Mah she'Chosav Oh she'Nodar): The Rosh says that he need not mention what he forbade that day. This is like a Tosefta (1:7), which says that 'this is upon me' forbids, even though he did not mention 'Korban'.

Gra (3 DH v'Chen): This is unlike Rashi (Shevuos 20b DH Taritz), who says that Rava holds that Hatfasah does not work even in Nedarim. He defines Hatfasah to be when he did not specify the Isur (I will not eat), rather, he said only 'this is like this.'

Taz (1): The Tur adds that 'it is forbidden to me' is also a Neder and one can be Matfis in it, even though he did not attribute it to Hekdesh.

Shulchan Aruch (3): If Levi heard Reuven vow and said 'and I am like you' Toch Kedei Dibur, he is forbidden what is forbidden to Reuven. If Shimon heard Levi and said 'and I', even if there are 100, and each said 'and I' Toch Kedei Dibur of the other, they are all forbidden.

Gra (6 and 239:23): This is like the Rambam. The Ran argues. We find Hatfasah after Kedei Dibur, e.g. 'I am like Shimshon' or 'eating meat is forbidden to me like the day that Gedalyah was killed.''

Shulchan Aruch (215:5): If one vowed about something forbidden such as eating a Neveilah, the Neder does not takes effect.

Shulchan Aruch (6): Some say that the Neder takes effect. If he eats it, he transgresses "Lo Yachel." However, if he was Matfis something else in it, it is not forbidden, for he intends for the initial Isur, which is Davar ha'Asur. However, if something is forbidden mid'Rabanan and he forbade it through a Neder and was Matfis something else in it, it is forbidden.