From the moment that the Holy Spirit begins to speak within the heart of the Christian, It preserves inside him a constant, incessant prayer:

"instead, be filled with the Spirit, saying (λαλούντες) to yourselves Psalms and hymns and spiritual odes, singing and chanting in your heart to the Lord, always thanking Him, for everything......"  [Ephesians 5:18-20]

"Praying in the Spirit..."   [Ephesians 6:19]

"...pray incessantly,.... thanking Him in everything... do not extinguish the Spirit, do not exhaust prophesy (προφητείας)..."  [1 Thessalonians 5:16-21]

That is the reason why in the Orthodox Church the "speaking in tongues" is called not only "the prayer of the heart", but also "incessant prayer".