Iago says that he is Satan himself. Where is this exact quotation in Shakespeare's Othello?

Throughout Othello, there are many
references to Iago being a devil or villian. Likewise, Christian theology of heaven and
hell blankets the text. There is an apparent battle between good and evil that is
embodied between God and the devil, who is most easily seen in the character
Iago.

I would note that the word Satan is not used. Devil
is often used by others, but in a conversation with with Cassio, Iago calls himself a
villian in this line:

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I am a very villain
else.

This line is spoken as
Iago is trying to learn if Cassio is going to marry Bianca. Iago uses this line after
telling Cassio that there are rumors flying about stating that Cassio and Bianca will
marry. Iago says this line meaning if the rumors aren't true, he is the devil himself.
This line occurs in Act IV, scene i.

Many characters allude
to hell or Iago as a devil or villain. Your question specifically asks where Iago calls
himself this being. This is the only spot I can find where he uses the word I, a verb
stating existence, and a synonym to your word Satan. Emilia calls him the "eternal
villian" in Act IV.ii, line 130. Othello calls him a devil in Act
V.

The first two links below will give you many more
references.