Mark 7:21-22
For from within, out of people's hearts, proceed evil thoughts, sexual immorality, stealing, murder, extramarital affairs,
Luke 6:45
The good person produces good things from the goodness stored up in his heart. And the evil person produces bad things from the evil [stored up in his heart]. For his mouth speaks what his heart is filled with.
James 1:26
If anyone considers himself to be religious and yet does not keep a tight rein on his tongue, this person is self-deceived and his religion is worthless.
James 3:5-8
So, the tongue is a small part [of the body], yet it boasts of great accomplishments. Look at how large a forest is burned up by even a tiny spark.