Answer:
In Act 2, scene 1 of Macbeth, he has a vision of a dagger floating in the air before him which he tries to grab at but fails because it is not real.
As Shakespeare wrote, "the night around him seems thick with horror and witchcraft, but Macbeth stiffens and resolves to do the bloody work".
His signal from Lady Macbeth came -a toll of the bell- which meant that the chamberlains were asleep and he could proceed to kill Duncan.
Shakespeare adds that as Macbeth murders Duncan, he thought he hear a cry that said "Sleep no more, / Macbeth does murder sleep” (2.2.33–34).