What quote shows that Macbeth is power-hungry in Act III?

To build on the quote the editor above uses, in
Shakespeare's Macbeth, Macbeth's worries about Banquo and Fleance
become more concrete and lead to plans to murder them in Act
3.2.

Macbeth is upset again, and his wife tries to calm him
down.  He tells her to give Banquo her special attention, because they need to fool
Banquo.  She tells him to stop talking like this, that Banquo and Fleance won't live
forever, and he responds:

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There's comfort yet; they are
assailable.

Then be thou jocund.  Ere the bat hath
flown

his cloistered flight, ere to black Hecate's
summons

The shard-borne beetle with his drowsy
hums

Hath rung night's yawning peal, there shall be
done

A deed of dreadful note. (Act
3.2.40-44)

Lady Macbeth,
then, trying to reassure him, says not to worry, Banquo and Fleance can't live forever,
and Macbeth turns that to something like:

• You're
  right, we should be happy.  They are reachable.  Before night falls, something dreadful
  will happen.

The something dreadful is
Macbeth's assassins trying to kill Banquo, and succeeding, and trying, but failing, to
kill Fleance.