1 Corinthians 14:2, 14
1 Corinthians 14:2 AMP
For one who speaks in an unknown tongue does not speak to people but to God; for no one understands him or catches his meaning, but by the Spirit he speaks mysteries [secret truths, hidden things].
1 Corinthians 14:14 AMP
For if I pray in a tongue, my spirit prays, but my mind is unproductive [because it does not understand what my spirit is praying].