What excuse does Lady Macbeth give for not killing Duncan herself in Shakespeare's Macbeth?

The episode you are referring to happens in Act II, Scenes 1 and 2 of Shakespeare's Macbeth.

In Act II, Scene 1, Macbeth decides that he will kill Duncan.  He hesitates a bit, giving his soliloquy about the knife and whether he really wants to go through with the murder, but when he hears the bell his wife rings he goes and kills Duncan.

Although Lady Macbeth wants Duncan dead (and although she provides the knife for Macbeth to use), she doesn't do it herself.  The excuse she gives is that Duncan, when sleeping, looks too much like her father.

I laid their daggers ready;
He could not miss 'em. Had he not resembled
My father as he slept, I had done't.