Musar for Shevuot 71:1

And, in truth, this is a wonder in my eyes. For it is the nature of a man to seek remedies and blessings from great men for success in [earning] a livelihood. And how will all the remedies and blessings help them if, G-d forbid, they are habituated to the sin of lashon hara and rechiluth, which is subject to an explicit curse [Arur] in the Torah (Devarim 27:24): "Cursed [Arur] is he who smites his friend in secret," which, according to Rashi, refers to lashon hara. And Chazal have said (Shevuoth 36a): "'Arur' — in it lies a curse; in it lies excommunication." And this [Arur] was not uttered by one man alone, but in consensus with all of Israel, in addition to the Cohanim and the Levites, as stated in Scripture. And this one [(by speaking lashon hara)] voided his blessing!

This is what the sages said in the Talmud (Shavuot 36a):"R. Eleazar said: 'No' is an oath; 'Yes' is an oath... Said Raba: But only if he said, 'No! No!' twice; or he said, 'Yes! Yes!' twice".