St. Aphrahat the Persian on prayer:

Why, my beloved, did our Saviour teach us saying: ‘Pray to your Father in secret, with the door shut’? I will show you, as far as I am capable. He said ‘Pray to your Father with the door closed’. Our Lord’s words thus tell us ‘pray in secret in your heart, and shut the door’. What is the door He says we must shut, if not your mouth? For here is the temple in which Christ dwells, just as the Apostle said: You are the temple of the Lord for Him to enter into your inner person, into this house, to cleanse it from everything that is unclean, while the door—that is to say, your mouth—is closed. If this were not the case, how would you understand the passage? Suppose you happened to be in the desert where there was no house and no door, would you be able to pray in secret? Or if you happened to be on top of a mountain, would you not be able to pray?

Source: Aaron Taylor at Logismoi.  The icon is by Lasha Kintsurashvili, and left to right it is mostly likely Ss. Ephraim and Isaac the Syrians and St. Aphrahat the Persian.