In which scene of the play did Othello grant Iago the lieutenantship?

In Act III, Scene III, Iago's lie about Desdemona's handkerchief comes to fruition. He tells Othello that he recently saw Cassio wiping his beard with the handkerchief, which convinces Othello without further doubt that his wife has indeed slept with Cassio. He vows to see Cassio and Desdemona dead. Iago promises to help him. Othello says Iago is now his lieutenant, and Iago says, "I am your own forever."