At least four times in the Gospels He is recorded as having spoken about how a good tree can only produce good fruit, and a bad tree can only produce bad fruit. He likens it to a good man only doing good things, and a bad man only doing bad things.
""... For out of the abundance of the heart the mouth speaks. ""
- Matthew 12.34
The mouth-heart/ tree-fruit thread is one that runs throughout the bible. What we are constantly reminded of is the fact that what is in our hearts is what is going to come out of our mouths.
"Guard your heart above all else, for it determines the course of your life"
- Proverbs 4.23
"...If any man does not stumble in word, he is a perfect man, able also to bridle the whole body."
- James 3.2
What is being said then, is that, until our hearts are in the right place, we won't be able to control what comes out of our mouths. This isn't something that is only relevant to a few of us, it's relevant to all of us, because all of us are human, with human hearts which host thoughts that God says are 'continually evil' (Genesis 6.5).
And so we get to the prayer of Psalm 51.10, where David, pleading with God, asks for a 'clean heart' and 'right spirit'. And that's where Jesus comes in. With Him came the Holy Spirit, the only spirit that is of God, and therefore wholly good. When we embrace God in all His fullness and allow the Holy Spirit to take root in our lives, how we live our lives becomes a result of being in and of God's Spirit; Love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control; all these being things that can be seen in all our daily actions, including our speech, so that instead of our behaviour dishonouring God, we praise Him with our whole heart (Psalm 9.1).
"Create in me a clean heart, O God,
and renew a right spirit within me"
- Psalm 51.10