Albert Barnes Commentary 1 Kings 1:29

"And the king sware, and said, As Jehovah liveth, who hath redeemed my soul out of all adversity," — 1 Kings 1:29 (ASV)

“As the Lord liveth” was the most common form of oath among the Israelites (for example, Judges 8:19; 1 Samuel 14:39; 1 Samuel 19:6). It was unique to David to attach a further clause to this oath—a clause of thankfulness for some special mercy (1 Samuel 25:34), or for God’s constant protection of him (here and in 2 Samuel 4:9).