Musar על מכות 46:1

And, especially, one who scrutinizes carefully the introduction to the Chafetz Chaim, and, the first two gates of all of its chapters will see that in fulfillment of the trait of guarding his tongue there will be deducted from him every year several hundreds and thousands of sins and added to him in exchange many hundreds and thousands of mitzvoth. For even if there only be deducted from him through this ten words of improper speech each day, that he has muzzled his mouth from speaking, it would still amount to over three thousand per year (And in this way I have explained what Chazal have said (Arachin 15b): "All who speak lashon hara increase sins until the heavens," the meaning being that if one is habituated to this sin, then from this itself his sins increase greatly), and those [muzzled] words would be reckoned as mitzvoth for him in the world to come. As Chazal have said (Makkoth 23b): "If a man sits [idle] and does not commit a sin, he is rewarded as one who has done a mitzvah, and, for every moment that he muzzles his mouth, etc." And he will also arrive, as a matter of course, at many holy traits, as I have written in the Second Gate of this work.