The Banquet scene, Act III, scene iv
Excerpt from the scene summary: Macbeth goes to sit at the head of the royal table but finds Banquo’s ghost sitting in his chair. Horror-struck, Macbeth speaks to the ghost, which is invisible to the rest of the company. Lady Macbeth makes excuses for her husband, saying that he occasionally has such “visions” and that the guests should simply ignore his behavior. Then she speaks to Macbeth, questioning his manhood and urging him to snap out of his trance. The ghost disappears, and Macbeth recovers, telling his company: “I have a strange infirmity which is nothing / To those that know me” (3.4.85–86). As he offers a toast to company, however, Banquo’s specter reappears and shocks Macbeth into further reckless outbursts. Continuing to make excuses for her husband, Lady Macbeth sends the alarmed guests out of the room as the ghost vanishes again.
Link to the full summary and information source: SparkNotes, "Macbeth" - http://www.sparknotes.com/shakespeare/macbeth/section6.rhtml
Image Source: Circulating Collection, Print and Picture Collection, Free Library of Philadelphia. Subject: "Macbeth" -- Shakespeare