Psalm 19:14 NKJV - "Let the words of my mouth and the meditation of my heart be acceptable in Your sight, O Lord, my strength and my Redeemer."
This is one of those great prayers in the Bible for a believer to meaningfully pray each morning. God knows our thoughts, Psalm 139:2, "...You understand my thought afar off." He also knows our words even before we say them! Psalm 139:4 NLT, "You know what I am going to say even before I say it, Lord." There's nothing hidden from the Lord, no matter how "quietly" we think or how softly we speak.
God not only hears the words from our voice as we speak, He also "hears" the words and thoughts on our mind, Jeremiah 17:10 - "I, the Lord, search the heart, I test the mind, even to give every man according to his ways, according to the fruit of his doings." Therefore, it is best that we seek to keep not only our words pure, but keep our hearts and mind pure as well.
Remember Proverbs 23:7, "For as he thinks in his heart, so is he..." The words of your lips and the actions of your heart spring from the wellspring of thoughts you have stored away in your mind. Note what Jesus said in Luke 6:45, "A good man out of the good treasure of his heart brings forth good; and an evil man out of the evil treasure of his heart brings forth evil. For out of the abundance of the heart his mouth speaks."
You want to "bring forth good" out of your life and lips? Then fill your heart and mind with the treasure of God's Word, Psalm 119:11, "Your word have I hid in my heart that I might not sin against You." God uses the Word in your heart to produce the Word in your life. And the Word in your heart also gives you plenty of good words to share in your conversations.