Commentary for Deuteronomy 17:1

לא תזבח ... כל דבר רע THOU SHALT NOT SACRIFICE [UNTO THE LORD THY GOD ANY OF THE HERD OR FLOCK WHEREIN IS BLEMISH OR] ANY EVIL THING — This is an admonition to one who would make sacrifices abominable through an evil utterance (דבר רע). (See Rashi on Leviticus 7:18; cf. Sifrei Devarim 147:5). Besides this, other Halachas have been derived from it in the Treatise on “The slaughtering of Sacrifices” (Zevachim 36).

THOU SHALT NOT SACRIFICE UNTO THE ETERNAL THY G-D [AN OX, OR A SHEEP, WHEREIN IS A BLEMISH], EVEN ANY ‘DAVAR’ (THING) THAT IS EVIL. “This is an admonition against causing offerings to be rendered unfit through an improper utterance.⁴⁹The expression davar ra (an evil thing) is thus interpreted to mean dibur ra — “an evil [or improper] utterance.” If the ministering priest has in mind [i.e., expresses] the intent to eat the meat of the offering, or to burn its assigned parts upon the altar, after the expiration of the prescribed times, he renders the entire offering unfit. See also Vol. III, p. 341, Note 84. There are yet other interpretations given by the Rabbis [on this verse] in the Tractate Shechithath Kadashim.”⁵⁰Literally: “The Slaughter of Hallowed Offerings,” as distinguished from Shechitath Chullin, “The Slaughter of Secular Animals.” The former is nowadays called Zebachim, “Animal Offerings,” while the latter is called Chullin (Secular Animals slaughtered for food). This is Rashi’s language. Now this is an explanatory commandment, for He has already admonished against slaughtering blemished offerings,⁵¹Leviticus 22:21. and here he repeated it in order to add the admonition against rendering it unfit by an utterance. It is likely that he repeated this admonition in order that the Israelite who brings the offering should not render it unfit during the slaughter [since non-priests were permitted to slaughter offerings]. The priests themselves, however, are scrupulous and careful, one warning sufficing for them. Therefore he did not repeat the prohibition against a blemished ministering priest [performing the Divine Service].

לא תזבח….אשר יהיה בו מום, "Do not slaughter for G'd anything blemished." The wording אשר יהיה בו מום "on which there will be a blemish," must be understood in conjunction with Sifri that our verse forbids an animal suffering from a temporary blemish. This ruling is hinted at by the word יהיה, "will be." That word includes not only permanent blemishes but even temporary blemishes. Perhaps one may also say that if the animal in question has a limb which would result in it becoming blemished unless said limb was amputated, the animal in question is considered blemished even before the operation (regardless of whether the animal is being considered for a sacrifice). The words לא תזבח …אשר יהיה בו מום mean "you must not sacrifice it…as it is going to develop a blemish" (which would disqualify it.)