Full Idea
I ought never to act except in such a way that I can also will that my maxim should become a universal law.
Gist of Idea
Act according to a maxim you can will as a universal law
Source
Immanuel Kant (Groundwork of the Metaphysic of Morals [1785], 402.17)
Book Reference
Kant,Immanuel: 'The Moral Law (Groundwork of Morals)', ed/tr. Paton,H.J. [Hutchinson 1948], p.67