Building the Altar with Stones Touched by Iron

The 79th prohibition is that we are forbidden from con­structing an altar1 from stones that have come in contact with metal.

The source of this mitzvah is G‑d's statement2 (exalted be He), "[When you build a stone altar for Me,] do not build it out of cut stone. Because your sword was lifted against it, [you have profaned it]."

If one builds an altar from such stones it is invalid and offerings may not be brought on it.

The details of this mitzvah are explained in the third chapter of tractate Middos.3