‘It is stated in 1 Cor. 14 that the chief use of tongues is in prayer. “For he that speaketh in an unknown tongue speaketh not unto man, but unto God: for no man understandeth him; howbeit in the Spirit he speaketh mysteries.” v. 2 “For if I pray in an unknown tongue, my spirit prayeth, but my understanding is unfruitful. What is it then? I will pray with the Spirit, and I will pray with the understanding also.” vv. 14, 15. The saints who love their Lord the most and who thrill to the delights of prayer will enter into the sentiments of Charles Wesley who sighed, “Oh, for a thousand tongues to sing my Great redeemer’s praise!” The gift of tongues is the answer to their prayer and deep heartdesire. The Spirit has provided the tongues of men and of angels  with which to pour out our hearts to God. “Likewise the Spirit also helpeth our infirmities: for we know not what we should pray for as we ought: but the Spirit himself maketh intercession for us with groanings which cannot be uttered.” Romans 8:26. Tongues are a God-ordained outlet through which the Spirit of Supplications will let His petitions ascend to the heavenly throne. (…)’
Source: Ralph M. Riggs, The Spirit Himself (1949), Chapter 22: Tongues and Interpretation of Tongues, p. 165.
 [note by crosstheology:] Why some Pentecostals call speaking in tongues “speaking in the tongues of angels” is unknown to the author of this website. Perhaps it is just an incorrect Pentecostal cliché.