Bava Metzia 84 - Robber Knows his Trade
Rabbi Yochanan, who was extremely beautiful, was swimming in the river. Resh Lakish, at this time a highway man, mistook him for a woman, and jumped after. Rabbi Yochanan told Resh Lakish that he had a sister even more beautiful than him, and if Resh Lakish decided to study Torah, he could marry her.
Resh Lakish became a great scholar. Once they were discussing when weapons are considered finished and therefore susceptible to ritual impurity. Rabbi Yochanan said, "when they are harderened," but Resh Lakish said "when they are polished." Rabbi Yochanan said, "Robber knows his trade" - as a praise, but Resh Lakish misunderstood and said, "And what have you given me? I was called Teacher among robbers also." Rabbi Yochanan was displeased, and Resh Lakish died. Rabbi Yochanan was inconsolable on the loss of his student and his mind became disturbed. The Sages asked for mercy on him, and he departed.