This was popular mythology among charismatics when I was among them. It was used to justify speaking in tongues as a "prayer language" (making interpretation unecessary): "My spirit prays, but my understanding is unfruitful (1st Corinthians 14:14)."

We described it in terms of the "infallible regenerate spirit praying" while the yet fallible and corruptible mind (or soul) did not profit from "praying in the spirit." The solution was to pray and sing "in the spirit, and with the mind also (verse 15)."

This was all based on the false assumption that the one who spoke in tongues couldn't understand his own speech. But both the Greek and the context of 1st Corinthians 14 very strongly imply that edification = understanding (with the mind) and that the goal of even "praying in the spirit" was to edify others.

-Robin