In davening we say a lot of pesukim from Torah and Nach, and there is a halacha that one should not say pesukim by heart. This is based on a Gemara in Masechta Temurah (14b) and Gittin (60b) that learns this from a darasha, (there is a discussion whether this is a real darasha or an asmachta (See Bircei Yosef O.C. 49:2)). The Ritva explains this halacha, that if we say pesukim by heart we will lose the derashos derived from the extra or missing words and letters and the similar.The Rambam and Shulchan Aruch write that when reciting a pasuk even one word is prohibited to be said by heart. (Rambam נשיאת כפיים 12:8, S.A. O.C. 144:1. See 139:3 even one letter).

Tomorrow we will discuss the opinion of the Mishna Berura.