Musar על בבא מציעא 87:5

We have explained this portion from the vantage point of sanctifying one's body and one's financial dealings. But the choicest part of sanctifying one's body is sanctifying the mouth. For this is what distinguishes a person from other living creatures, such that he is called a speaking being: It is written (Genesis 2:7), "and man became a living being" - and Onkelos (the classic Aramaic translation of the Torah) renders this as, "man (Adam) became a talking spirit." From this stems the duty of a person to fulfill everything that comes out of his mouth, as it is stated (Deuteronomy 23:24) "You must keep what has come out of your lips." And even though this is mentioned in connection with vows, that is as an additional warning [not to default on vows]. Regardless, it is also stated about everything. For our external speech flows from our inner speech, and that is the speech that God and the angels use. And it is this speech that God uses when speaking with us, as it is stated [in many places], "And God spoke, etc." - and [there,] it is written, "saying," to be a warning to us that our inner speech come from our souls which is a portion of God above. Hence, all of a person's toil should be towards his mouth. Woe to him and to his soul of the one who defiles himself with foul language or lies or that which is similar to them. Rather, care must be taken in avoiding evil and guarding the mouth even in mundane conversation. Truly one should only open one's mouth in praise and thanksgiving to God, with the voice of Torah [study] and of prayer. And then, 'the voice will be the voice of Jacob,' and the external speech will flow from the inner [speech]. But the generation of the flood, as well as the generation of the [Tower of Babel], did not stand by their inner speech. Hence anyone who does not stand by his word must accept upon himself [the curse of (Bava Metzia 44a),] "He Who exacted payment from the people of the generation of the flood, and from the generation of [the Tower of Babel], will exact payment [from whoever does not stand by his statement]."