SARAH JANE: You’re serious, aren’t you?

THE DOCTOR: About what I do, yes. Not necessarily the way I do it.

Doctor Who (1963-1989) British science fiction television series, original run (BBC)

11×01 “The Time Warrior,” Part 3 (1973-12-29) [w. Robert Holmes]
    (Source)

(Source (Video)). Often Sarah Jane's line is misquoted as "Doctor, are you serious?"