"This shall be the law for a leper" - this is what is written (Psalms 36:13), "Who is the man who desires life?" There is a story of a peddler who would go around to towns that were close to Tzippori. He would shout out and say, "Who wants to buy the [potion] of life?" They would [all] cling to him. Rabbi Yannai was sitting and interpreting [texts] in his reception room [and] heard him shouting out, "Who wants to buy the [potion] of life?" [Rabbi Yannai] said, "Come down to here, sell [it] to me." He said [back] to him, "You do not need it and those like you do not [need it]." [Nonetheless,] he made the effort to come and go down to him. He took out a book of Psalms and showed him the verse, "Who is the man who desires life?" [The peddler said,] "What is written after it - 'guard your tongue from evil [...] Turn away from evil and do good' (Psalms 34:14-15)." Rabbi Yannai said, "Shlomo also shouted out and said (Proverbs 21:23), 'He who guards his mouth and his tongue, guards his soul from troubles.'" Rabbi Yannai said "All of my days I was reading this verse and I did not know how to interpret it until this peddler came and made it understood - 'Who is the man who desires life?'" Therefore, Moshe warns Israel and says to them, "This shall be the law for a leper (metsora)" - the law of the one that gives out a bad name (motsee shem).