WIST

There is, therefore, only one categorical imperative. It is: Act only according to that maxim by which you can at the same time will that it should become a universal law.

Immanuel Kant (1724-1804) German philosopher
Groundwork of the Metaphysics of Morals [Grundlegung zur Metaphysik der Sitten], Sec. 2 (1785) [tr. Beck (1969)]

Alternate translations:

1. I ought never to act except in such a way that I could also will that my maxim should become a universal law.
2. Act only on that maxim which you can at the same time will that it should become a universal law.
3. Act as if the maxim of thy action were to become by thy will a universal law of nature.
4. So act that your principle of action might safely be made a law for the whole world.
5. May you live your life as if the maxim of your actions were to become universal law.
6. Live your life as though your every act were to become a universal law.
7. Do not feel forced to act, as you're only willing to act according to your own universal laws. And that's good. For only willful acts are universal. And that's your maxim.

(As noted in the comments, the "alternate translations" may represent other restatements by Kant of the categorical imperative.)