In the Large Catechism, Martin Luther expounds the Third Commandment (“Remember the Sabbath Day by keeping it holy”) by extolling the treasure of the Lord’s Word. One of the way the Lord’s Christians honor the Word is by memorizing it. Here’s what Luther says about that:

Look at the grip Luther has on that Bible.

Know, therefore, that you must be concerned not only about hearing, but also about learning and retaining it in memory, and do not think that it is optional with you or of no great importance, but that it is God’s commandment, who will require of you how you have heard, learned, and honored His Word. (LC I.98)