Charles Ellicott Commentary 2 Samuel 24:18

"And Gad came that day to David, and said unto him, Go up, rear an altar unto Jehovah in the threshing-floor of Araunah the Jebusite." — 2 Samuel 24:18 (ASV)

Gad came. — As appears from 1 Chronicles 21:18, by direction of the angel. Daniel was still in Jerusalem proper, i.e., the hill of Zion, and it was while looking out from there that he had seen the angel “by the threshing-floor of Araunah,” i.e., on the lower hill of Mount Moriah, which afterwards became the site of the Temple, and was included within the city. It was doubtless this event that determined the Temple-site.