וַיָּבֹא־גָ֥ד אֶל־דָּוִ֖ד בַּיּ֣וֹם הַה֑וּא וַיֹּ֣אמֶר ל֗וֹ עֲלֵה֙ הָקֵ֤ם לַֽיהוָה֙ מִזְבֵּ֔חַ בְּגֹ֖רֶן ארניה [אֲרַ֥וְנָה] הַיְבֻסִֽי׃
And Gad came that day to David, and said unto him: ‘Go up, rear an altar unto the LORD in the threshing-floor of Araunah the Jebusite.’
Read Kabbalistic (mystical) interpretations of II Samuel 24:18: Zohar and classical Jewish mystical sources, free online.