In Act 1 Scene III, Banquo is horrified when he catches a glimpse of the ugly witches. He comments that the witches don't look like beings from earth and questions whether they are really women since they sport beards. Meanwhile, Macbeth demands the witches explain what sort of creatures they are. The witches ignore his command; instead, the first witch hails Macbeth as the thane of Glamis, and the second witch hails him as the thane of Cawdor. The third witch hails him as the future king.
Macbeth is stunned, as he is not yet the thane of Cawdor. After all, the thane of Cawdor is still alive and a 'prosperous gentleman' to boot. The first witch also proclaims that Banquo will both be lesser and greater than Macbeth. The second witch tells Banquo that he will encounter the seemingly paradoxical experience of not being as happy as Macbeth while being happier than him. The third witch lets on that Banquo will never be king, but that his progeny will be. None of the witches answers Macbeth's questions about how they came about their prophecies.
When Ross later tells Macbeth that the thane of Cawdor has been sentenced to death for his treason and that Macbeth is to be the new thane of Cawdor, Macbeth is amazed that the witches have been right all along. He is so happy that the witches' prophecies have come true that he wants to know what Banquo thinks. Banquo, however, is cautious, warning Macbeth that
oftentimes, to win us to our harm,The instruments of darkness tell us truths,Win us with honest trifles, to betray ’sIn deepest consequence.