Babylonian Talmud Bava Kamma 2a
Regarding Shabbat, we learned: “The primary prohibited acts (avot melakhah) are forty less one” (Mishnah Shabbat 7:2). “Primary prohibited acts” implies that there must be secondary prohibited acts (toldot). The secondary acts are the same as [the primary ones]; there is no difference between a primary and a secondary [prohibited act] . . .