Charles Spurgeon Commentary 1 Chronicles 21:18-20

"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. And David went up at the saying of Gad, which he spake in the name of Jehovah. And Ornan turned back, and saw the angel; and his four sons that were with him hid themselves. Now Ornan was threshing wheat." — 1 Chronicles 21:18-20 (ASV)

Then the angel of the LORD commanded Gad to say to David, that David should go up, and set up an altar unto the LORD in the threshingfloor of Ornan the Jebusite. And David went up at the saying of Gad, which he spake in the name of the LORD. And Ornan turned back, and saw the angel;

He was busy at his threshing, and he saw the angel standing by his own threshing floor.