Here's Luther from the Large Catechism:
But all depends upon this, that we learn also to say "Amen." This means that we do not doubt that our prayer is surely heard and that what we pray shall be done (2 Corinthians 1:20). This is nothing else than the word of undoubting faith, which does not pray on a dare but knows that God does not lie to him (Titus 1:2). For He has promised to grant it. Therefore, where there is no such faith, there cannot be true prayer either.
