Large Catechism: Prayerful Duty (LC III 1-11)

The first thing to know is that it is our duty to pray because of God's commandment. For that's what we heard in the Second Commandment, "You shall not take the name of the LORD your God in vain" [Exodus 20:7]. We are required to praise that holy name and call upon it in every need, or to pray. To call upon God's name is nothing other than to pray [e.g., 1 Kings 18:24]. (5)