Temurah 16A

אמר רב יוסף: דופי של סמיכה קתני.

Rav Yosef said in response: The baraita is not referring to a flaw due to some sin; rather, it is teaching about the flaw of the early dispute over the halakha of placing hands on the head of an animal brought as a Festival peace offering, as taught in tractate Ḥagiga (16a).

והא יוסף בן יועזר גופיה מיפליג פליג בסמיכה! כי איפליג בה בסוף שניה, דבצר ליבא.

The Talmud raises a difficulty: But Yosef ben Yo’ezer himself disputed the halakha of placing hands on the head of an offering. The first rabbis to dispute this issue were Yosef ben Yo’ezer and Yosef ben Yoḥanan. The Talmud answers: When they disputed it, that was at the end of the years of Yosef ben Yo’ezer’s life, when the understanding of his heart was limited, due to old age. Therefore, the dispute is considered as though it occurred after his lifetime.

גופא, אמר רב יהודה אמר שמואל: שלשת אלפים הלכות נשתכחו בימי אבלו של משה. אמרו לו ליהושע: שאל! אמר להם: ״לא בשמים היא״.

The Talmud returns to the matter itself. Rav Yehuda says that Shmuel says: 3,000 halakhot were forgotten during the days of mourning for Moses. The Jewish people said to Joshua: Ask for guidance from Heaven so that you can reacquire the forgotten halakhot. Joshua said to them: “It is not in heaven” (Deuteronomy 30:12). Once the Torah was given on Sinai, the rabbis of each generation must determine the halakha. No new halakhot may be added or subtracted by heavenly instruction or through prophecy.

אמרו לו לשמואל: שאל! אמר להם: ״אלה המצות״⁠ — שאין הנביא רשאי לחדש דבר מעתה. אמר רבי יצחק נפחא: אף חטאת שמתו בעליה נשתכחה בימי אבלו של משה.

Many years later the Jewish people again said to Samuel: Ask for halakhic guidance from Heaven. He said to them: This is not possible, as the Torah states: “These are the commandments that YHWH commanded Moses to tell the children of Israel at Mount Sinai” (Leviticus 27:34). The word “these” indicates that from now on a prophet is not permitted to introduce any new element related to the Torah and its mitzvot through prophecy. With regard to the topic of the chapter, R' Yitzḥak Nappaḥa says: Also, the halakha of a sin offering whose owner has died was one of those forgotten during the days of mourning for Moses.