Pray without ceasing. – 1 Thessalonians 5:17

What do these words imply? “Pray without ceasing.” Do they not imply that the use of the voice is not an essential element in prayer? It would be most unseemly even if it were possible for us to continue unceasingly to pray aloud. There would of course be no opportunity for preaching and hearing, for the exchange of friendly intercourse, for business, or for any other of the duties of life; while the din of so many voices would remind our neighbors rather of the worship of Baal than that of Zion. It was never the design of the Lord Jesus that our throats, lungs, and tongues should be for ever at work. Since we are to pray without ceasing, and yet could not pray with the voice without ceasing, it is clear that audible language is not essential to prayer. We may speak a thousand words which seem to be prayer, and yet never pray; on the other hand, we may cry into God’s ear most effectually, and yet never say a word.

It is equally clear that the posture of prayer is of no great importance, for if it were necessary that we should pray on our knees we could not pray without ceasing, the posture would become painful and injurious…It is well to pray on one’s knees; it is a most fitting posture; it is one which expresses humility, and when humility is truly felt, kneeling is a natural and beautiful token of it, but, at the same time, good men have prayed flat upon their faces, have prayed sitting, have prayed standing, have prayed in any posture, and the posture does not enter into the essence of prayer. Consent not to be placed in bondage by those to whom the bended knee is reckoned of more importance than the contrite heart. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/1039.cfm