Othello Act 3, Scene 4: "The same."
Desdemona asks the Clown about the whereabouts of Cassio's lodgings. The Clown runs her around in verbose circles. She frets to Emilia about the loss of her beloved handkerchief. Othello enters, immediately grilling his wife with questions concerning her loyalty. He requests the handkerchief and is aghast when she cannot give him the exact one for which he asks. He says that there is ancient Egyptian magic in it, and that the loss of it tells strongly against their love. Othello exits abruptly as Iago and Cassio enter. Desdemona tells Cassio that she is trying to win her husband's affections for him, but that he is enraged. Iago quickly leaves to find Othello. Emilia tells Desdemona that her husband is insanely jealous. "But jealous souls will not be answered so; / They are not ever jealous for the cause, / But jealous because they're jealous. It is a monster / Begotten upon itself, born on itself" Act 3, Scene 4.
As Desdemona and Emilia exit the citadel, Cassio's mistress, Bianca, enters, seeking her lover. She becomes bitter and jealous of Cassio's relationship with Desdemona and finds the handkerchief lying in his room. Cassio says he found the handkerchief and liked the pattern; he gives Desdemona's handkerchief to Bianca asking her to copy the pattern.