Foucault discussed Kant’s essay ‘What is Enlightenment?’ in various places.

DE = Dits et écrits; EW = Essential Works; PT = The Politics of Truth

• lecture of 27 May 1978, published as “Qu’est-ce que la critique? (Critique et Aufklärung)”, Bulletin de la Société française de philosophie, Vol 84 No 2, 1990. Not in DE; translations in PT and The Essential Foucault, among others.
• “Introduction”, Georges Canguilhem, The Normal and the Pathological, Boston, D. Reidel, 1978, pp. ix-xx; DE no 219 III, 429-42.
• “Pour une morale de l’inconfort”, Le nouvel observateur, No 754, 1979, pp. 82-3; republished as DE no 266 III, 783-7; translations as “For an Ethics of Discomfort” in PT and EW III.
• 5 January 1983, opening lecture of The Government of Self and Others, now published in full; excerpt previously available as ‘Un cours inédit’, Magazine littéraire, No 207, May 1984, pp. 35-39 (reprinted in DE no 351 IV, 679-688); translated by Colin Gordon as “Kant on Enlightenment and Revolution”, Economy and Society, Vol 15 No 1, 1986, pp. 88-96; various reprints/different translations including PT.
• “What is Enlightenment?” in The Foucault Reader, 1984, pp. 32-50; French in DE no 339 IV, 562-78, reprinted in EW I, PT, and others; French/English parallel texts at foucault.info
• “La vie: l’expérience et la science”, Revue de métaphysique et de morale, Vol 90 No 1, 1985, pp. 3-14; reprinted in DE no 361; revised form of DE no 219; translated in EW II.

The essay is mentioned in other places, but I think these are the major references.

The secondary literature is enormous. A good place to start is James Schmidt’s collection What is Enlightenment?: Eighteenth Century Answers and Twentieth Century Questions and his blog, Persistent Enlightenment.

More Foucault resources – a few short translations, bibliographies, links, comparisons of texts, etc. – available here.