Jamieson, Fausset and Brown on 1 Chronicles 21:26

26. David built there an altar--He went in procession with his leading men from the royal palace, down Mount Zion, and through the intervening city. Although he had plenty of space on his own property, he was commanded, under peremptory direction, to go a considerable distance from his home, up Mount Moriah, to erect an altar on premises which he had to buy. It was on or close to the spot where Abraham had offered up Isaac.

answered him by fire from heaven--(See Le 9:24; 1Ki 18:21-23; 2Ki 1:12; 2Ch 7:1).