It is significant that Macbeth echoes the witches’ lines as his first line. This ties him to the witches, and demonstrates their influence on him.
When Macbeth first speaks, he seems to repeat the witches’ warning from the first scene.
So foul and fair a day I have not seen. (Act 1, Scene 3, p. 12)
By repeating what the witches say, Macbeth is demonstrating that he is possibly influenced by them. Later on in the play, Hecate scolds them for messing with him. They seem to already be playing with him. By beginning the play with the witches, Shakespeare is foreshadowing the dark and supernatural elements. By having Macbeth echo this line, we see that the witches are at work.