出エジプト記 20:22

ואם מזבח אבנים AND IF (according to Rashi, AND WHEN) [THOU WILT MAKE ME] AN ALTAR OF STONES — Rabbi Ishmael said: Every time the word אם it used in the Torah it refer to some action the doing of which is optional, except in three instances. Here: ואם מזבח אבנים תעשה לי — you see that this אם is used in the sense of כאשר, “when”, the meaning being: “And when thou makest me an altar of stone, לא תבנה אתהן גזית THOU SHALT NOT BUILD IT OF HEWN STONE, for, as a matter of fact, it is obligatory upon thee to build an altar of stone, as it is said, (Deuteronomy 27:6) “of whole stones thou shalt build [the altar of the Lord thy God]”. Similarly, (Exodus 22:24) אם כסף תלוה את עמי is obligatory and signifies: “when (אם) thou lendest My people money”, and not, “if thou lendest”, because it is said, (Deuteronomy 15:8) “And thou shalt surely lend him”; consequently this אם also is used in the sense of כאשר “when”. Similarly, (Leviticus 2:14) ואם תקריב מנחת בכורים, for this refers to the meal-offering of the Omer which is obligatory and therefore the trantlation is, “And when (ואם) thou offerest the meal-offering of first-fruits”, and not, “if thou offerest”. You must therefore admit that these instances of אם are not conditional, meaning “if”, but they are absolute, and that they are used in the sense of כאשר, “when” (Mekhilta; Yalkut Shimoni on Torah 306; cf. Rashi on Leviticus 2:14). גזית — This has the meaning of cutting (גזיזה), the stones being thus called because one hews them and cuts them with an iron tool (Mekhilta). כי חרבך הנפת עליה — This word כי is used here in the sense of פן, which signifies “perhaps it may be” (and it does not mean “because”) — “perhaps it may happen that thou liftest up thy iron tool above it”. ותחללה THEN THOU HAST PROFANED IT — Thus you may learn that if thou liftest up thy iron tool above it thou profanest it. The reason of this is, because the altar is created (its purpose is) to lengthen man’s days and iron has been created (one of its purposes is) to shorten man’s days, it is not right that an object which shortens man’s life should be lifted up above that which lengthens it (Mekhilta, Middoth 3:4). And a further reason is: because the altar makes peace between Israel and their Father in Heaven, and therefore there should not come upon it anything that cuts and destroys. Now, the following statement follows logically, à fortiori: How is it in the case of stones which cannot see nor hear nor speak? Because that they promote peace Scripture ordains, “Thou shalt not lift up against them any iron tool!” Then in the case of one who makes peace between a man and his wife, between family and family, between a man and his fellow, how much more certain is it that punishment will not come upon him (Mekhilta).

וְאִם־מִזְבַּ֤ח אֲבָנִים֙ תַּֽעֲשֶׂה־לִּ֔י לֹֽא־תִבְנֶ֥ה אֶתְהֶ֖ן גָּזִ֑ית כִּ֧י חַרְבְּךָ֛ הֵנַ֥פְתָּ עָלֶ֖יהָ וַתְּחַֽלְלֶֽהָ׃

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 defiled it.