Exodus 20:25
And if you make me an altar of stone do not make it of cut stones: for the touch of an instrument will make it unclean.
Deuteronomy 27:5-6
There you are to make an altar to the Lord your God, of stones on which no iron instrument has been used.
Joshua 8:31
In the way ordered by Moses, the servant of the Lord, as it is recorded in the book of the law of Moses, an altar of uncut stones, untouched by any iron instrument: and on it they made burned offerings and peace-offerings to the Lord.
Treasury of Scripture Knowledge
Summary
And if thou wilt make me an altar of stone, thou shalt not build it of hewn stone: for if thou lift up thy tool upon it, thou hast polluted it.
General references
Bible References
And
Deuteronomy 27:5
There you are to make an altar to the Lord your God, of stones on which no iron instrument has been used.
Joshua 8:31
In the way ordered by Moses, the servant of the Lord, as it is recorded in the book of the law of Moses, an altar of uncut stones, untouched by any iron instrument: and on it they made burned offerings and peace-offerings to the Lord.
General references
Leviticus 16:33
And he will make the holy place and the Tent of meeting and the altar free from sin; he will take away sin from the priests and from all the people.