Shakespeare's "MacBeth", though I believe that's actually from two different phrases in the play (rather than being one continuous one).
EDIT: Yep: Act 1, Scene 5:
15 Glamis thou art, and Cawdor; and shalt be
16 What thou art promised. Yet do I fear thy nature;
17 It is too full o' the milk of human kindness
18 To catch the nearest way. Thou wouldst be great;