Who said Life is but a walking shadow?

The phrase "Life is but a walking shadow" is from William Shakespeare's play "Macbeth." It appears in Act 5, Scene 5, spoken by Macbeth himself as he reflects on the futility and transience of life after learning of Lady Macbeth's death. The line conveys a sense of despair and the idea that life is insubstantial and fleeting, much like a shadow.