Commentaire de citation sur Jérémie 9:4

"on truth": As a person should walk in the paths of repentance - as He is true and His Torah is true - and walk in the ways of the Holy One, blessed be He, of truth. And he should also go in that path, as it is stated (Deuteronomy 28:9), "and you shall go in His ways." And the sages, may their memory be blessed, said (Yevamot 63a) that even in the recounting of mundane words should a person not lie, like that story of Rav's son, etc.: [Rav] said to him, "Your mother has improved." He said [back] to him, "It is I that reversed [your words to bring about the change]." He said, "This is as people say, 'The one that comes from you will teach you reason.' But you should not do this because of that which is stated (Jeremiah 9:4), 'they taught their mouths to speak words of falsehood.'" For a person who accustoms his tongue to speak falsehood about a matter that has neither a loss nor gain will - when he comes to speak matters of principle - also not be able to speak the truth; as it is his mouth that will speak and habit rules over it.