During Talmudic times, one of the most disturbing elements in the Jewish community was a moser – someone who collaborated with the foreign government, informing on his fellow Jews. Such a person was perceived as being dangerous to the well-being of the society at large. This led to a ruling by the Sages that a moser could be killed in order to protect the community.
In an illustration of this rule, the Gemara tells of someone who was warned by Rav that he could not inform the government about his neighbor’s straw (as we have learned, tax collectors were seen as generally dishonest during this period, having purchased the right to collect taxes according to their own whims), but who insisted that he was going to do so, in any case. Rav Kahane who was present and saw the interaction between them, stepped forward and killed the man, given his insistence of playing the role of a moser. Recognizing the danger that Rav Kahane now faced as an accused murderer, Rav pointed out to him that with the passing of the Parthian rulers (who were lax in their rule, and allowed autonomous rule for minorities) and the rise of the Sassanian Dynasty (who were much more involved in the day-to-day lives of their subjects), Rav Kahane could be accused of murder. Rav suggested that Rav Kahane move to Israel and study with the great Israeli amora, Rabbi Yohanan, but insisted that he accept upon himself not to ask him questions for a period of seven years.
The Maharsha explains that Rav’s insistence of Rav Kahane’s acceptance of this limitation on his study may have been a type of penance – not for the act of killing the moser, but for ruling about this case in the presence of his teacher. Therefore, an appropriate penance would be to refrain from asking questions, even in areas that were not clear rulings. The Iyyun Yaakov suggests otherwise, arguing that Rav was simply instructing his student to accept the traditions of study in Israel, where the accepted method involved discussion rather than questioning.
Upon his arrival in Israel, Rav Kahane impressed the students in Rabbi Yohanan’s academy, but when he was given a prize seat in the front of the classroom, heeding Rav’s advice, he did not participate in the class discussion. Upon being demoted seven rows, Rav Kahane felt that the demotion was equivalent to the seven years that his teacher had ruled he should remain silent in class, and he once again began to question and offer answers in the classroom.