While Desdemona sleeps, Act V, scene ii
Excerpt from a scene summary: Desdemona lies asleep in bed, and Othello enters, dreadfully calm and sure in what he must do. Desdemona wakens and calls him to bed, but he tells her to pray at once, repenting anything she needs to repent, and he will wait while she prays because he does not want to kill her soul. Suddenly, Desdemona realizes that Othello intends to kill her. She is afraid, although she knows she is not guilty. Knowing that she cannot convince him of her fidelity, Desdemona weeps and begs him to banish her rather than kill her, or let her live just a little more, but he stifles her, presumably with a pillow.
Link to the full summary and information source: CliffNotes, "Othello" - http://www.cliffsnotes.com/literature/o/othello/summary-and-analysis/act-v-scene-2
Image Source: Circulating Collection, Print and Picture Collection, Free Library of Philadelphia. Subject: "Othello" – Shakespeare
Original source: Engraved by E. Buchel from a painting by H. Hofmann, New York: Patterson & Neilson Richard III