In act 2 of Othello, what lie does Iago tell Montano about Cassio?

In Act 2 Scene 3 of the play Othello by William Shakespeare, Iago is setting the wheels in motion for his convoluted plan to destroy Othello. He needs to do this in a very convoluted fashion, going all round the houses to get what he wants - the final outcome where all his prepared and primed evil jigsaw pieces fall into place. In this scene, he is trying to discredit Cassio and knows that reputation is all and a bad one gets round to gossipers and bosses alike. Little by little he will destroy Cassio's credibilty as a tried, tested and true comrade of Othello's. He starts by lying about a "drink problem." We can see the unlikeliness of this in Montano's incredulous reply at first "But is he often thus?" He wants to give him the benefit of the doubt that this is just a one-off, but Iago persists, saying he is (in effect) an alcoholic or drink-dependent who can't even sleep if "drink rock not his cradle."