2 Samuel 7:18

We have here the solemn address David made to God, in answer to the gracious message God had sent him. We are not told what he said to Nathan; no doubt he received him very kindly and respectfully as God's messenger. But his answer to God he took himself, and did not send by Nathan. When ministers deliver God's message to us, it is not to them, but to God, that our hearts must reply; he understands the language of the heart, and to him we may come boldly. David had no sooner received the message than, while the impressions of it were fresh, he retired to return an answer. Observe, I. The place he retired to: He went in before the Lord, that is, into the tabernacle where the ark was, which was the token of God's presence; before that he presented himself. God's will now is that men pray everywhere; but, wherever we pray, we must set ourselves as before the Lord and set him before us. II. The posture he put himself into: He sat before the Lord. 1. It denotes the posture of his body. Kneeling or standing is certainly the most proper gesture to be used in prayer; but the Jews, from this instance, say, "It was allowed to the kings of the house of David to sit in the temple, and to no other.

— Matthew Henry's Concise Commentary (public domain)

DAVID'S PRAYER AND THANKSGIVING. (Sa2 7:18-29) Then went king David in, and sat before the Lord--Sitting was anciently an attitude for worship (Exo 17:12; Sa1 4:13; Kg1 19:4). As to the particular attitude David sat, most probably, upon his heels. It was the posture of the ancient Egyptians before the shrines; it is the posture of deepest respect before a superior in the East. Persons of highest dignity sit thus when they do sit in the presence of kings and it is the only sitting attitude assumed by the modern Mohammedans in their places and rites of devotion.

— Jamieson-Fausset-Brown Commentary (public domain)