Musar על קידושין 78:15

For is it not written (Mishlei 2:4-5): "If you seek for it as silver, if you search for it as hidden treasure — then you will understand the fear of the L-rd." And it is well known that everyone would prefer having a steady income even with less profit than a chance income with greater profit. How much more so with this ["income" of guarding one's tongue], which is a steady income with great profit. For with this mitzvah one can earn both when he is sitting in his house or when he is in the house of prayer or the house of study, and also when he goes to the market and sees people speaking what they should not be speaking (and distancing himself from them.) As Chazal have said (Kiddushin 39b): "If one sat and did not transgress, he is rewarded as the doer of a mitzvah." In sum: With this mitzvah one can "earn" from the time he rises from his sleep [in the morning] until the time he goes to sleep in the evening, effortlessly, and with great profit. For, for every moment of muzzling his mouth he merits the "secreted light," as Chazal have stated.