Book 11 of Aulus Gellius preserves a delightful remark by Cato the Elder from the lost Carmen de Moribus:
Indeed, human life is very like iron. If you use it, it wears out; if you do not, it is nevertheless consumed by rust. In the same way we see men worn out by toil; if you toil not, sluggishness and torpor are more injurious than toil.
Thanks to Bill Thayer for making this available online.