At the end of act 1, scene 7 of William Shakespeare's Macbeth, Macbeth resolves to murder King Duncan and take the throne of Scotland for himself in order to fulfill the prophecy made to him by the three witches in act 1, scene 3.
Macbeth's last lines in act 1, scene 7—"Away, and mock the time with fairest show: / False face must hide what the false heart doth know"—seem to be addressed to Lady Macbeth but are, in fact, addressed to himself.
Macbeth has no need to remind Lady Macbeth to be duplicitous, since she said essentially the same thing to Macbeth earlier in the play, only in different words:
LADY MACBETH. To beguile the time,
Look like the time; bear welcome in your eye,
Your hand, your tongue; look like the innocent flower,
But be the serpent under't.
(act 1, scene 5, lines 68–71)
"Look like the innocent flower, / But be the serpent under't" and "False face must hide what the false heart doth know" both mean for Macbeth and Lady Macbeth to appear to the world as if they have no evil intentions towards Duncan but to remain steadfast in their decision to kill him.
Macbeth has occasion to put his own words into practice almost immediately. In the next scene, Macbeth is on his way to Duncan's chambers to kill him when he encounters Banquo and Banquo's son, Fleance, on midnight watch. This must have been unnerving for Macbeth, who's still not entirely convinced that he's doing the right thing by killing Duncan, which is evidenced by his "Is this a dagger which I see before me" soliloquy later in the scene (lines 41–69).
Banquo also reminds Macbeth about the prophecies of the witches—one prophecy saying that Macbeth "shalt be King hereafter!"—which Macbeth intends to fulfill within the next few minutes.
BANQUO. I dreamt last night of the three weird sisters:
To you they have show'd some truth.
(act 2, scene 1, lines 24–25)
Macbeth responds to Banquo with a lie, saying, "I think not of them," which is the first time since he decided to kill Duncan that Macbeth acts like an innocent flower and hides his false heart with a false face.