Pesachim 42A

רבי יהודה היא, דתניא: ״שור ושה שרוע וקלוט נדבה תעשה אתו״. אותו אתה מתפיס לבדק הבית, ואי אתה מתפיס תמימים לבדק הבית. מכאן אמרו: כל המתפיס תמימים לבדק הבית עובר בעשה.

It is in accordance with the opinion of R' Yehuda, as it was taught in a baraita: “Either a bull or a lamb that has anything too long or too short, you may offer it as a free-will offering [to the Temple treasury]; but for a vow [as a sacrifice] it should not be accepted” (Leviticus 22:23). From here we learn that it, i.e., a blemished animal, you may consecrate for maintaining the Temple, but you may not consecrate unblemished animals for maintaining the Temple. In other words, any animal fit to be sacrificed as an offering may not be consecrated for maintaining the Temple but only as an offering. From here the rabbis stated: Whoever consecrates unblemished animals for maintaining the Temple transgresses a positive mitzva.

אין לי אלא בעשה, בלא תעשה מנין? תלמוד לומר: ״וידבר ה׳ אל משה לאמר״, לימד על כל הפרשה כולה שיהא בלא תעשה, דברי רבי יהודה.

From here I have only derived that he violates a positive mitzva; from where do I derive that he also transgresses a prohibition? The verse states at the beginning of that passage: “And YHWH spoke to Moses saying” (Leviticus 22:17). This introductory statement teaches with regard to the entire portion that a prohibition applies to it. This is the statement of R' Yehuda.

אמר לו רבי לבר קפרא: מאי משמע?

The baraita adds that R' Yehuda HaNasi said to Bar Kappara: From where may it be inferred that this is the case? How does R' Yehuda derive his statement that a prohibition applies to the entire portion from the phrase “And YHWH spoke to Moses saying”?

אמר לו: דכתיב ״לאמר״⁠ — לא נאמר בדברים.

He said to him: As it is written: “Saying [leimor].” R' Yehuda expounds this term as though it read: Say no [lo emor]. In other words, the word no, an expression of prohibition, is stated with regard to the subsequent matters, which means that these mitzvot are categorized as prohibitions.