Matthew 12:34
You children of snakes! Since you are evil, how do you expect to speak good things? For the mouth speaks what the heart is filled with.
Romans 3:13-14
[Psa. 5:9 says], "Their throats are like open graves [i.e., they are foul-mouthed]. They have spoken deceitfully." [Psa. 90:3 says], "There is snake venom under their lips [i.e., their words are deadly]."
James 3:6-8
And the tongue is [like] a fire [i.e., like that spark]. [For] the tongue, as a part of the body, is [like] the world of evil. It corrupts the entire life [of a person] and inflames the course of nature itself [i.e., it affects a person throughout his whole earthly existence], and is ignited by hell itself [i.e., the evil produced by the fiery tongue has hell as its source and destiny].