Question: who tells macbeth to beware of macduff?
Answer: The Witches warn Macbeth to "beware Macduff, beware the Thane of Fife" (4.1. 81–82). However, they inform Macbeth that, "The power of man, for none of woman born / Shall harm Macbeth" (4.1. 91–92) — leading one to infer that no human could possibly defeat Macbeth.