Albert Barnes Commentary 1 Chronicles 21:16

"And David lifted up his eyes, and saw the angel of Jehovah standing between earth and heaven, having a drawn sword in his hand stretched out over Jerusalem. Then David and the elders, clothed in sackcloth, fell upon their faces." — 1 Chronicles 21:16 (ASV)

Here a picture of awe-inspiring grandeur takes the place of the bare statement of the earlier historian in 2 Samuel 24:17. And here, as elsewhere, the author probably extracts from the ancient documents such circumstances as harmonize with his general plan. As the sanctity of the temple was among the points on which he was most anxious to lay stress, he gives in full all the miraculous circumstances attending this first designation of what became the temple site (marginal reference “k”) as a place “holy to the Lord.”

David and the elders ... clothed in sackcloth, fell upon their faces—These are facts additional to the narrative of Samuel, but they are natural in themselves and in harmony with that account. Similarly, the narrative in 1 Chronicles 21:20 is additional to the account in Samuel, but its parts are coherent, and there is no sufficient ground for suspecting it.