Sounds like you're thinking of the https://en.wikipedia.org/wiki/Is%E2%80%93ought_problem . Wikipedia quotes David Hume's "A Treatise of Human Nature" 3.1.1 as follows:

> In every system of morality which I have hitherto met with [...] the author proceeds for some time in the ordinary way of reasoning [...] when of a sudden I am surprised to find that instead of the usual copulations of propositions is and is not I meet with no proposition that is not connected with an ought or an ought not.