What does Iago say that finally makes Othello doubtful in Shakespeare's Othello?Act III, Sc. 3

Iago taunts Othello throughout this scene. Firstly he
implies through repeating his master's words that all is not as it seems with Cassio. He
then goes on to warn Othello, in an act of dramatic irony, to counsel his own
thoughts:

O,
beware, my lord, of jealousy! It is the green-eyed monster, which doth mock The meat it
feeds on. That cuckold lives in bliss Who, certain of his fate, loves not his wronger;
But O, what damned minutes tells he o'er Who dotes, yet doubts, suspects, yet strongly
loves!

We as an audience can
see that Othello is already unsettled by this idea. Iago then goes further, confessing
to Othello that he witnessed a dream of Cassio's which implicated him in a relationship
with Desdemona:

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In sleep I heard him say, “Sweet Desdemona,

Let us be wary, let us hide our
loves”;

Othello is wracked to
torment, and demands that Iago has real proof. As we know he has planted Desdemona's
handkerchief in Cassio's chamber, we see Iago use his ultimate weapon--a form of 'proof'
that will drive Othello to kill--

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I know not that; but such a handkerchief—

I am sure it was your wife's—did I today(485)
See Cassio wipe his
beard with.

The significance
of the handkerchief--ironically the most flimsy of evidence--is as Othello's first gift
to Desdemona.  Iago has, at this point, succeeded in convincing Othello of his wife's
infidelity.