Rabbi Eliezer taught: It is prohibited for a disciple to issue halakhic rulings before his teacher until he is at a distance of twelve mil from him, corresponding to the camp of Israel. That is what is written: “They encamped along the Jordan, [from Beit Yeshimot until Avel Shitim]” (Numbers 33:49). How much is that? It is twelve mil.
Rabbi Tanḥum ben Rabbi Yirmeya was in Ḥefer. They would ask him and he would issue halakhic rulings. They said to him: ‘Did you not teach us in the study hall that it is prohibited for a disciple to issue halakhic rulings before his teacher until twelve mil, and is your teacher, Rabbi Manei, not in Tzippori?’ He said to them: ‘May [punishment] come upon me if I knew.’29He swore to them that he did not know his teacher was within twelve mil. From that time he did not issue halakhic rulings.