John Gill Commentary 1 Chronicles 21:15

"And God sent an angel unto Jerusalem to destroy it: and as he was about to destroy, Jehovah beheld, and he repented him of the evil, and said to the destroying angel, It is enough; now stay thy hand. And the angel of Jehovah was standing by the threshing-floor of Ornan the Jebusite." — 1 Chronicles 21:15 (ASV)

(See Gill on 1 Chronicles 21:1).