There isn't one single Bible verse that says "think before you speak because the mind of the Lord instructs his mouth" verbatim. However, several verses emphasize the importance of thoughtful speech and aligning our words with God's wisdom. Here are a few relevant examples:
* Proverbs 16:23: "The heart of the wise makes their speech prudent, and their lips promote instruction."
* Proverbs 15:28: "The heart of the righteous weighs its answers, but the mouth of the wicked gushes evil."
* Proverbs 10:19: "When words are many, sin is not absent, but he who holds his tongue is wise."
* James 1:19: "My dear brothers and sisters, take note of this: Everyone should be quick to listen, slow to speak and slow to become angry."
These verses suggest that wise and righteous people carefully consider their words, understanding the power they hold. They imply that a thoughtful heart, guided by wisdom (which comes from God), will lead to prudent speech. While not a direct quote, the idea that our words should be guided by wisdom and consideration aligns with the concept you described.