4 But let our speech and petition when we pray be under discipline, observing quietness and modesty. Let us consider that we are standing in God's sight. We must please the divine eyes both with the habit of body and with the measure of voice. For as it is characteristic of a shameless man to be noisy with his cries, so, on the other hand, it is fitting to the modest man to pray with moderated petitions. Moreover, in His teaching the Lord has bidden us to pray in secret— in hidden and remote places, in our very bed-chambers— which is best suited to faith, that we may know that God is everywhere present, and hears and sees all, and in the plenitude of His majesty penetrates even into hidden and secret places, as it is written, “I am a God at hand, and not a God afar off.
If a man shall hide himself in secret places, shall I not then see him? Do not I fill heaven and earth?” And again: “The eyes of the Lord are in every place, beholding the evil and the good.” And when we meet together with the brethren in one place, and celebrate divine sacrifices with God's priest, we ought to be mindful of modesty and discipline— not to throw abroad our prayers indiscriminately, with unsubdued voices, nor to cast to God with tumultuous wordiness a petition that ought to be commended to God by modesty; for God is the hearer, not of the voice, but of the heart.
Nor need He be clamorously reminded, since He sees men's thoughts, as the Lord proves to us when He says, “Why do you think evil in your hearts?” And in another place: “And all the churches shall know that I am He that searches the hearts and reins.”
Source: The Treatises of Cyprian (New Advent)