-- Saint Bonaventure, also quoted by Fr. Flavian Walsh, O.F.M., in a pamphlet published by the Franciscan Friars' Holy Name Province.
"True prayer is found not in the sound of the voice, but in the desire of the heart. Not our words, but our desires give power and meaning to our prayers. If we ask for eternal life with our lips without desiring it from the bottom of our hearts, our cry is silence. If without speaking, we desire it from the bottom of our heart, our silence cries out."
The photo above is from here.