Charles Ellicott Commentary 1 Chronicles 21:18

"Then the angel of Jehovah commanded Gad to say to David, that David should go up, and rear an altar unto Jehovah in the threshing-floor of Ornan the Jebusite." — 1 Chronicles 21:18 (ASV)

Then the angel of the Lord commanded Gad to say to David. —Rather, Now the angel had told Gad to tell David. In Samuel, the mediation of the angel is not mentioned. There we read, And Gad came that day to David, and said to him, Go up, etc.

No doubt it is only in the later prophetical books of the Canon that angels are introduced as the medium of communication between God and His prophets. (See Daniel 8:16, 9:21; Zechariah 1:9, 1:12, and following; but compare to Judges 6:11, 6:14, 6:16, and following, and Genesis 18:1-2, 18:13, 32:24, 32:30.)