Jamieson, Fausset and Brown on 1 Chronicles 21:25

25. David gave ... for the place six hundred shekels of gold--At first he bought only the cattle and the threshing instruments, for which he paid fifty shekels of silver (2Sa 24:24); afterwards he purchased the whole property, Mount Moriah, on which the future temple stood. High in the center of the mountain platform rises a remarkable rock, now covered by the dome of "the Sakrah." It is irregular in its form, and measures about sixty feet in one direction and fifty feet in the other. It is the natural surface of Mount Moriah and is thought by many to be the rock of the threshing-floor of Araunah, selected by David, and continued by Solomon and Zerubbabel as "the unhewn stone" on which to build the altar [BARTLETT, Walks about Jerusalem; Stanley].