In Macbeth, do you think "So foul and fair a day I have not yet seen" is just a literary device used to allude to the witches spell?
I do think there seems to be a very direct correlation between Macbeth's statement "so foul and fair a day I have not seen" to the witches comment "Fair is foul and foul is fair." Between the ominous setting of the first scene and the witches' language, we associate evil doings with the witches.
The repetition of this sentiment links Macbeth to the witches and allows the audience to connect the evil connotations associated with the witches to Macbeth. The paradox "fair is foul and foul is fair" illustrates that things that seem on the surface good may actually be foul. Macbeth acts as if he is good to the public throughout the entire play but underneath, he embodies that of a foul, rotten king.