1st Corinthians 14:12

9 so also ye, if through the tongue, speech easily understood ye may not give -- how shall that which is spoken be known? for ye shall be speaking to air.

10 There are, it may be, so many kinds of voices in the world, and none of them is unmeaning,

11 if, then, I do not know the power of the voice, I shall be to him who is speaking a foreigner, and he who is speaking, is to me a foreigner;

12 so also ye, since ye are earnestly desirous of spiritual gifts, for the building up of the assembly seek that ye may abound;

13 wherefore he who is speaking in an [unknown] tongue -- let him pray that he may interpret;

14 for if I pray in an [unknown] tongue, my spirit doth pray, and my understanding is unfruitful.

15 What then is it? I will pray with the spirit, and I will pray also with the understanding; I will sing psalms with the spirit, and I will sing psalms also with the understanding;