Book V of the Wealth of Nations has a long passage about the ill effects of the division of labor (or more specifically, of the constant repetition of simple tasks) on the workers.
Cheers, that was a good read. I'd still attribute the argument to Marx, because Smith doesn't seem to link this effect with the inherent values of capitalism.
Turns out there's also a wikipedia page outlining the history of this line of reasoning:
Here: https://www.marxists.org/reference/archive/smith-adam/works/... (search for "In the progress of the division of labour")