Musar על ערכין 29:14

Moreover, one who is accustomed to speak gossip will sometimes speak words against God, as it is written, "They have set their mouth against the heavens, and their tongue walketh through the earth" (Ps. 73:9). And there is no greater guilt in all of the sins than the guilt of him who blasphemes God. And our Sages, of blessed memory, said, "With ten trials were our ancestors tried, and in all them their fate was not sealed except for the sin of gossip" (Arakin 15a, Shohar Tov, 39a). As it is said, "As ye have spoken in mine ears, so will I do to you" (Num. 14:28). And it says, "And the Lord heard the voice of your words and was wroth" (Deut. 1:34). And the Torah itself does not protect gossipers. Doeg the Edomite, as soon as he had spoken gossip, his wisdom did not avail him and all of his knowledge of the Torah did not protect him (Sotah 21a, Sanh. 106b).