Who in Macbeth said "I am in blood, Stepp'd in so far, that, should I wade no more, Returning were as tedious as go o'er"?

This quote is from Macbeth himself, in Act III, Scene 4. It occurs just after he has excused his guests at the dinner party after seeing the recently-murdered Banquo's ghost, and is spoken to his wife. What it means, more or less, is that Macbeth views himself at a point of no return. He has murdered both Duncan and Banquo by this point, and is concerned that Macduff did not show up for the dinner (a concern that will be vindicated by the witches later). He has "stepp'd" so deep into murder and treachery that he cannot get out of it--it is as difficult ("tedious") to turn back as to forge ahead. He plans to consult with the witches to get advice, and the passage suggests is that Macbeth will commit even more murderous deeds to maintain and consolidate his position of power.