“I ought never to act except in such a way that I could also will that my maxim should become a universal law.”

Der kategorische Imperativ, der überhaupt nur aussagt, was Verbindlichkeit sei, ist: handle nach einer Maxime, welche zugleich als ein allgemeines Gesetz gelten kann.
Source: Metaphysics of Morals (1797), Ch. 11

England's Ideal: And Other Papers on Social Subjects (1887) p. 54

Midnight Palace Interview: Mamie Van Doren, Written by Gary Sweeney http://www.midnightpalace.com/index.php?option=com_content&view=article&id=334:interview-mamie-van-doren

"The Summing Up: Meredith Dawe," Do What You Will (1970)

The Physics of Star Trek, HarperPerennial edition (1996), p. 16.

The Enchiridion (c. 135)
Context: Never call yourself a philosopher, nor talk a great deal among the unlearned about theorems, but act conformably to them. Thus, at an entertainment, don't talk how persons ought to eat, but eat as you ought. For remember that in this manner Socrates also universally avoided all ostentation. And when persons came to him and desired to be recommended by him to philosophers, he took and recommended them, so well did he bear being overlooked. So that if ever any talk should happen among the unlearned concerning philosophic theorems, be you, for the most part, silent. For there is great danger in immediately throwing out what you have not digested. And, if anyone tells you that you know nothing, and you are not nettled at it, then you may be sure that you have begun your business. For sheep don't throw up the grass to show the shepherds how much they have eaten; but, inwardly digesting their food, they outwardly produce wool and milk. Thus, therefore, do you likewise not show theorems to the unlearned, but the actions produced by them after they have been digested. (46).

Source: The Production of Security (1849), p. 25

Groundwork of the Metaphysics of Morals (1785)
Context: Morality is thus the relation of actions to the autonomy of the will, that is, to a possible giving of universal law through its maxims. An action that can coexist with the autonomy of the will is permitted; one that does not accord with it is forbidden. A will whose maxims necessarily harmonize with the laws of autonomy is a holy, absolutely good will. The dependence upon the principle of autonomy of a will that is not absolutely good (moral necessitation) is obligation. This, accordingly, cannot be attributed to a holy being. The objective of an action from obligation is called duty.