וּבָנִ֤יתָ שָּׁם֙ מִזְבֵּ֔חַ לַיהוָ֖ה אֱלֹהֶ֑יךָ מִזְבַּ֣ח אֲבָנִ֔ים לֹא־תָנִ֥יף עֲלֵיהֶ֖ם בַּרְזֶֽל׃
And there shalt thou build an altar unto the LORD thy God, an altar of stones; thou shalt lift up no iron tool upon them.
Read kabbalistic (mystical) interpretation of Deuteronomy 27:5: and classical mystical sources.