Gemara

(Beraisa) Suggestion: If a father told his son (a Kohen) to become Tamei (Mes), or not to return an Aveidah, perhaps he should obey!

Rejection: "Ish Imo v'Aviv Tira'u v'Shabsosai Tishmoru" - you are all obliged to honor Me.

Inference: If not for "Ish Imo v'Aviv Tira'u...", we would say that he should listen!

Question: Kivud Av v'Em is an Aseh. An Aseh does not override a Lav and an Aseh!

Answer: Honor of parents is equated to honor of Hash-m. It says "Kaved Es Avicha v'Es Imecha", and "Kaved Es Hash-m". Therefore, one might have thought that he should listen. The verse teaches that he should not.

Rishonim

Rif (16b): The Aseh of Kivud Av v'Em does not override the Lav (for neglecting) and Aseh of Hashavas Aveidah.

Rashba (32a DH Yachol): We must say that the father asked his son to serve him. If not, there is no Aseh of Kivud Av v'Em to override Hashavas Aveidah! There is an Aseh of Hashavas Aveidah only if he already picked it up. The Gemara could have answered that the verse is needed for when he did not pick it up. The Gemara answered that in every case one would have thought that Kivud Av v'Em overrides. The primary Perush is that the Gemara asked about an Aseh and Lav of Tum'ah.

Rif and Rosh (17b and 2:28): One might have thought that if a father told his son to become Tamei, or not to return a lost object, he should obey. "Ish Imo v'Aviv Tira'u v'Shabsosai Tishmoru (Ani Hash-m Elokeichem)" - you are all obliged to honor Me.

Nimukei Yosef (DH Gemara): We learn from "Ani Hash-m."

Rambam (Hilchos Gezeilah v'Aveidah 11:19): If one saw an Aveidah and his father told him not to return it, he returns it and does not obey. If he would obey, while fulfilling Kivud Av he would Mevatel the Aseh "Hashev Teshivem" and the Lav "Lo Suchal Lehis'alem".

Magid Mishneh: This is even if the father tells the son not to return it so the son can honor him by bringing to him food. The Rambam holds like the Rif, that if he does not return, he transgresses also a Lav. The primary Perush holds that the Lav is not for an Aveidah (rather, for Tum'as Kohen). This does not affect the Halachah.

Taz (CM 266:5): This is only if the Mitzvah of Hashavas Aveidah occurred and then his father told him not to return it. If he was already engaged in serving his father, he is exempt from Hashavas Aveidah, even if his father did not mention the Aveidah. One engaged in a Mitzvah is exempt even from an Aseh accompanied by a Lav. A Shomer Aveidah caring for the Aveidah is exempt from giving Tzedakah, which is accompanied by a Lav. This is not only because there is also a Lav with Hashavas Aveidah (but there is no Lav about Kivud Av v'Em). The Rif (16b) taught that the Lav does not help to override other Mitzvos. However, if the father is here, he is obligated to pardon his honor so the son can return it, or the father himself must return it, for also the father must honor Hash-m. However, there is no obligation on the son as long as he is engaged in Kivud Av v'Em. Do not say that the son is not exempt because the father himself is obligated. If so, also a Shomer Aveidah would be obligated to give Tzedakah!

Mishneh l'Melech: Had the father told the son not to return (without a proper reason, merely) to transgress, obviously he would not obey, for the father is a Rasha!

Rebuttal (Michtam l'David YD 33): The father does not cease to be "Amcha" for just telling his son not to return it. A Rasha is one who transgressed a Lav for which one is lashed (CM 34). Even if the father transgressed Lifnei Iver, there are no lashes, for there is no action. It seems that he did not transgress Lifnei Iver, for he can say that he was testing his son. He is not a Mesis (enticer to sin). The son should obey the Rav, not the Talmid! If he transgressed, he caused his own loss.

Rambam (Hilchos Mamrim 6:12): If one's father told him to transgress Torah, be it a Lav or an Aseh or mid'Rabanan, he may not obey. "Ish Imo v'Aviv Tira'u v'Shabsosai Tishmoru" - all of you must honor Me!

Sefer ha'Mitzvos (Aseh 112, cited in Hagahos Maimoniyos 8): This shows that one does not become a Rasha through speech (telling someone to transgress). This is because the listener should obey the Rebbi (Hash-m), not the Talmid. If the father became a Rasha for this, we would not need a verse to teach that one does not obey! The only one punished for such speech is one who entices to serve idolatry.

Rambam (12): If one's father told him 'bring to me water', and there is another Mitzvah to do, if someone else can do the other Mitzvah, the son lets them do it and serves his father. If not, he does the Mitzvah and abandons Kivud Av v'Em, for also his father is obligated in the Mitzvah.

Michtam l'David (ibid.): In Hilchos Aveidah, the Rambam connotes that he must return only because there is a Lav and an Aseh. Here, he says that one does not obey his father to transgress even a mid'Rabanan! Me'il Shmuel says that there the Rambam discusses when the father said 'serve me, and do not return it.' Here, he discusses when the father told him to transgress for no reason, for then there is no Mitzvah of Kivud Av v'Em. This is wrong. The Gemara concluded 'you are all obliged to honor Me.' This teaches that one may not transgress even a Lav mid'Rabanan. If one's father and mother both requested water, he honors the father, for also his mother must honor his father! In Halachah 13, the Rambam discusses when the Mitzvos of Kivud Av v'Em and Hashavas Aveidah come to the son at the same time (Kesef Mishneh), and he rules that one serves his father if someone else can do the other Mitzvah. In Hilchos Aveidah, since there is also a Lav, the son must return the Aveidah even if someone else could.

Question (Kesav v'Chosam R. Akiva Eiger 30a Tosfos DH Ha): The Gemara said that Kivud Av v'Em, an Aseh, does not override a Lav and an Aseh. Rashi explains that this refers to Hashavas Aveidah. Tosfos explains that it refers to Tum'as Kohanim. Presumably, Tosfos did not say that it refers to Hashavas Aveidah, because this would not need a verse. Monetary matters do not override Isurim. However, the Riva (in Tosfos Chulin 141a DH Lo) says that even though an Aseh is not Docheh a Lav and an Aseh, it is Docheh lashes for the Lav. If so, why does the Gemara question the need for a verse? If not for the verse, we would say that Kivud Av v'Em is Docheh the lashes for Tum'ah! This is not difficult for Rashi, for in any case one is not lashed for not returning an Aveidah, since Ein Bo Ma'aseh. (Still, we could ask why the Gemara asked. Perhaps the Tana holds that one is lashed for Lav she'Ein Bo Ma'aseh!)

Answer #1 (ibid.): Since we know that it is not Docheh, just one might have thought that the Aseh exempts from lashes, the Beraisa should have taught that Kivud Av is not even Docheh a plain Lav.

Answer #2 (ibid.): Even if one who tells a stranger to transgress is a Rasha, there was a Hava Amina that Kivud Av v'Em overrides Isurim. If it did, one would be permitted to tell his child to transgress. Therefore, in any case we need a verse!

Poskim

Shulchan Aruch (YD 240:15): If one's father told him to transgress Torah, be it a Lav or an Aseh or mid'Rabanan, he may not obey.

Gra (26): Yevamos 6a connotes that only Kares is not Nidcheh for Kivud Av v'Em. However, the Medrash Rabah (14:6) brings this after teaching that we do not follow a king's command to transgress, even though (normally) one who violates the king's command is Chayav Misah. We do not transgress any Devar Torah to honor a king, all the more so for Kivud Av v'Em!

Shulchan Aruch (CM 266:5): If one's father told him not to return an Aveidah, he does not obey.