Charles Ellicott Commentary Luke 12:39-40

"But know this, that if the master of the house had known in what hour the thief was coming, he would have watched, and not have left his house to be broken through. Be ye also ready: for in an hour that ye think not the Son of man cometh." — Luke 12:39-40 (ASV)

And this know, that if the goodman of the house ...—Better, "if the master of the house." See Notes on Matthew 24:43-44, where the words are almost identical.