1 Corinthians 14:9

So also you, unless you utter by the tongue speech that is clear, how will it be known what is spoken? For you will be speaking into the air.

1 Corinthians 9:26

Therefore I run in such a way, as not without aim; I box in such a way, as not beating the air;

Treasury of Scripture Knowledge

Summary

So likewise ye, except ye utter by the tongue words easy to be understood, how shall it be known what is spoken? for ye shall speak into the air.

Bible References

Easy

1 Corinthians 14:19
however, in the church I desire to speak five words with my mind so that I may instruct others also, rather than ten thousand words in a tongue.

For

1 Corinthians 9:26
Therefore I run in such a way, as not without aim; I box in such a way, as not beating the air;