How does Macbeth's first line, "So foul and fair a day I have not seen" establish a foreboding atmosphere?

This line echoes the Witches' incantation of the end of
the first scene of the play:

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Fair is foul and foul is
fair:

Hover through the fog and filthy
air.

So, for Macbeth to echo
these words makes him appear to somehow be aligned with the Witches, which casts an evil
and foreboding atmosphere around him.

The evil association
would simply have come from the social assumption of the general population of
Shakespeare's day that witches existed to perform evil deeds (the work of the Devil). 
Shakespeare didn't need to prove the evil of the witches to his audience, they would
simply have assumed it.  So, Macbeth's very first impression on the audience links him
to the characters that represent evil in the play.

It is
also an eerie foreboding that is created when Macbeth utters the words that the Witches
have incanted.  The suggestion that they have the power and ability to affect the future
is implanted in the audiences mind at this moment, adding to the ominous foreboding of
the atmosphere of Act One.

Please follow the links below
for more on the Witches and how Elizabethans saw them and evil in the
play.