Midrasch zu Schemot 38:8

When the Holy One, blessed be He, told Moses to build the Tabernacle, all the Israelites brought their contributions. Some brought silver, others brought gold or copper or onyx stones or unset stones. They brought everything eagerly. The women asked themselves: What contribution can we make to the Sanctuary? They arose, took their mirrors, and brought them to Moses. When Moses saw them he became angry with them. He said to the Israelites: Take your canes and beat them on their shoulders. What purpose do these mirrors serve? The Holy One, blessed be He, called out to Moses: Moses, do you mistreat them because of these? These very mirrors produced the hosts in Egypt. Take them and make a basin of brass and its base for the priests, that they may sanctify the priests from it, as it is said: And he made the laver of brass, and base thereof of brass, of the mirrors of the serving women that did service (ibid. 38:8), for they had produced all the hosts. Therefore it is written: And the brass of the offering was seventy talents (ibid., v. 29), that is, the offering of the wives amounted to seventy talents.