Someone recently mentioned to me a story that in the days of the Tannaim most people learned from a single teacher and learned a single derekh or set of halakhot. Rabbi Yehudah HaNasi was unusual in that he went around to all of the yeshivot to gather their teachings. And there was one school where they did not want to teach him because they said (in essence) we only teach our Master's torah to people who are going to keep it and not mix it with other torah. Does anyone know the source of this story? And does anyone know how it ends?