Musar for Avodah Zarah 40:9

To what may this be compared? To a groom who is being brought by his groomsmen to the marriage canopy, and, on the way, is accosted by some empty fellows who bespatter him with mud and mire from the soles of his feet until his head. When the groomsmen shout at them: "Empty ones, where did you get so much filth?" they show all [who are assembled there] that the groom is one of their companions and that he himself prepared all the mud and mire. So is it, exactly, in our instance. A man himself, through the filth of his sins, creates this revolting, unclean vestment, and, perforce, dons it and cannot rid himself of it, since he himself prepared it. As Scripture states (Isaiah 50:11): "Walk in the flame of your fire and in the brands that you have kindled. By My hand has this come to you." And Chazal have said, similarly (Avodah Zarah 20b): "Let a man not think [lewd thoughts] in the daytime and come to uncleanliness at night." They have hereby taught us that if he had not first put the thought into his mind, the spirits of uncleanliness could not have cleaved to him and defiled him.