This teaching is found in the third volume of the Zohar (53a).
 The following teaching is found in maseches "Yoma" (39a): It was taught in the Academy of Rabbi Yishmael: "Sin makes the heart of man foolish, as it says, "And do not become impure from them and you shall be made impure in their midst", do not read "and you shall become impure" - "V'Nitmaisem", rather read "and you shall be made stupid" - "V'Nitamtem"." Our rabbis taught [in a Baraisa], "[If] a person makes himself a little impure, they make him very impure. [If he makes himself impure] from Below, [in this world], they make him impure from Above, [in Heaven]. [If he makes himself impure] in this world, he is made impure in The World to Come." Our rabbis have taught [in a Baraisa]: ""And you shall makes yourselves holy and you shall become holy" - [If] a person makes himself a little holy, is is then made very holy. [If he makes himself holy] from Below, [in this world], they make him holy from Above, [in Heaven]. [If he makes himself holy] in this world, they make him holy in The World to Come."