I was first alerted to this by Matt Zwolinski of Bleeding Heart Libertarians. It comes from Adam Smith, An Inquiry Into the Nature and Causes of the Wealth of Nations:
The property which every man has in his own labour, as it is the original foundation of all other property, so it is the most sacred and inviolable. The patrimony of a poor man lies in the strength and dexterity of his hands; and to hinder him from employing this strength and dexterity in what manner he thinks proper without injury to his neighbour, is a plain violation of this most sacred property. It is a manifest encroachment upon the just liberty both of the workman, and of those who might be disposed to employ him. As it hinders the one from working at what he thinks proper, so it hinders the others from employing whom they think proper. To judge whether he is fit to be employed, may surely be trusted to the discretion of the employers whose interest it so much concerns. The affected anxiety of the law–giver lest they should employ an improper person, is evidently as impertinent as it is oppressive.