מדרש אויף דברים 27:6

Similarly, (Exodus 20:22) "And im ("when") an altar of stones you make unto Me." It is mandatory. You say it is mandatory, but perhaps it is optional? (Devarim 27:6) "Of whole stones shall you build the altar of your G–d" indicates that it is mandatory and not optional.

8) R. Yochanan b. Zakkai said (Devarim 27:6) "Of whole (shleimoth) stones shall you build the altar of the L–rd your G d, etc.": "stones which make peace" (shalem). Now does this not follow a fortiori: If stones, which do not see, and which do not hear, and which do not speak — because they "make peace" between Israel and their Father in heaven, Scripture writes (Devarim 27:5) "You shall not lift iron over them," then a man who makes peace between a man and his wife, between family and family, between city and city, between country and country, and between one people and another — how much more so will calamity not befall him!

Similarly, "And if an altar of stones you make for Me." This is mandatory. You say it is mandatory, but perhaps it is optional. (This is not so,) for it is written (Devarim 27:6) "Of whole stones shall you build the altar of the L–rd." It is mandatory and not optional (to build an altar). And what is the intent of "if an altar of stones"? If he wishes to build it of stones, he may do so. (If he wishes to build it) of bricks, he may do so. Now does this not follow a fortiori for the other vessels, viz. If he may vary (i.e., either stones or bricks) for the altar, the more "formidable," how much more so may he do so for other vessels (of lesser formidability) (e.g., the menorah and the table, to make them of silver.)