“Finally, mark this, that you must always speak the Amen firmly. Never doubt that God in his mercy will surely hear you and say “yes” to your prayers. Never think that you are kneeling or standing alone, rather think that the whole of Christendom, all devout Christians, are standing there beside you and you are standing among them in a common, united petition which God cannot disdain. Do not leave your prayer without having said or thought, ‘Very well, God has heard my prayer; this I know as a certainty and a truth.’ That is what Amen means.” Quoted in Taking Hold of God: Reformed and Puritan Perspectives on Prayer, eds. Joel R. Beeke and Brian J. Najapfour (Grand Rapids: RHB, 2011), 16.
The opening chapter on Luther’s teaching on prayer is excellent.