וּמַלְאַ֧ךְ יְהוָ֛ה אָמַ֥ר אֶל־גָּ֖ד לֵאמֹ֣ר לְדָוִ֑יד כִּ֣י ׀ יַעֲלֶ֣ה דָוִ֗יד לְהָקִ֤ים מִזְבֵּ֙חַ֙ לַיהוָ֔ה בְּגֹ֖רֶן אָרְנָ֥ן הַיְבֻסִֽי׃
Then the angel of the LORD commanded Gad to say to David, that David should go up, and rear an altar unto the LORD in the threshing-floor of Ornan the Jebusite.
Read midrashic interpretation of I Chronicles 21:18: and classical rabbinic exegesis.