9780899067087 - Schottenstein Bavli Shabbos volume 1, published by Mesorah

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xxxix: With the exception of the prohibition to kindle a fire, though, the Torah does not explicitly state what constitutes a labor.
1. Even for kindling the Torah does not explicitly say that it is forbidden because it is a labor (Exodus 35.3).
2. In Exodus 16 Israel is restrained from gathering manna on the sabbath so this can be added to the Torah's definition of labor.
3. Exodus 34.21 says וביום השביעי תשבת בחריש ובקציר תשבת so we can add plowing and harvesting to the Torah's definition of labor.
4. In Numbers 15, a man is executed for gathering wood on the sabbath so this too can be added to the Torah's definition of labor.