Charles Ellicott Commentary 1 Chronicles 21:28

"At that time, when David saw that Jehovah had answered him in the threshing-floor of Ornan the Jebusite, then he sacrificed there." — 1 Chronicles 21:28 (ASV)

At that time when David saw ... —The use of Ornan’s threshingfloor as a place of sacrifice was continued from the time of the cessation of the pestilence. The words then he sacrificed there refer to this fact. The answer by fire from heaven (1 Chronicles 21:26) was an unmistakable intimation of the Divine will that it should be so. (Compare to Joshua 5:15.)