Midrash for Sanhedrin 200:16

The Alphabet is apparently the work of a late author who pored over the book of Ben Sira and selected from it several sayings and several other things that were said in his name in the Aramaic language which was then the common language among the masses, and organized them alphabetically as a mnemonic aid. And after him came a second author who made a second alphabet, in the Holy Tongue, because he found some Talmudic and Midrashic content, and appended it to them in order to complete the alphabet. And after him came a third author, and appended to these a commentary

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:

י) "The watchman does not sleep. When she is a minor- lest she be seduced or raped in her youth. When she has grown up- lest she commit adultery." (see Sanhedrin 100b) He said to him, "Everything that you have said, you have said truly." He said to him, "Say 'Kaf.'" He said to him: