מדרש על סנהדרין 200:8

ה) "Blind your eyes because of the graceful woman, lest you be caught in her trap." The educator said to him, "My son, what trap could I be caught in? If it is because of some magic you could do for me, I know that you will never do magic for me! Because her first husband is a thin-bearded man, (see Sanhedrin 100b and Rashi there) and I am a thick-bearded man." He said to him, "Say 'Vav.'" He said to him: