Charles Ellicott Commentary 1 Chronicles 17:26

"And now, O Jehovah, thou art God, and hast promised this good thing unto thy servant:" — 1 Chronicles 17:26 (ASV)

Lord, thou art God. — Jehovah, You are the (true) God. Samuel: “my Lord Jehovah.” The chronicler omits the clause which follows in Samuel: “and thy words become truth” (prove true).