1 Corinthians 14:6

But now, brethren, if I come unto you speaking with tongues, what shall I profit you, unless I speak to you either by way of revelation, or of knowledge, or of prophesying, or of teaching.

What shall I profit you ... means "I shall not profit you in any manner at all," if I come to you speaking in tongues. This was Paul's refusal to speak in tongues in the Christian assembly at Corinth; and it is safe to assume that he never did so anywhere else. The only way that even an apostle could benefit his hearers was by preaching to them.

By way of revelation ... refers to what was revealed in Scripture.

"Or of knowledge" refers to the spiritual gift of knowledge which Paul assuredly had.

Or of prophesying ... refers to intelligible teachings given by the Holy Spirit to Paul as a spiritual gift.

Or of teaching ... refers to ordinary teaching of what was learned from others, orally or through study of their writings.

Here again the essentially private nature of the true gift of tongues is implicit and demanded by the context.