Shakespeare uses frequent imagery and irony to foreshadow Macbeth's killing Banquo.
There is much blood imagery in the play; though, we don't see the murder of Duncan, it has been described vividly, as have the deaths of his guards. Here, Macbeth is trying to transfer some of the blame (and blood) onto Malcolm and Donaldbain, the "bloody cousins." This is also ironic since, of course, Duncan's sons are innocent of the crime.
There is also much animal imagery in Macbeth; it has been reported that horses have eaten each other on the night of the murder. Here, we have talk of horses, not so much symbolic of anyone in particular, but Shakespeare includes it to subvert the natural order (the Great Chain of Being). Macbeth's statement, "I commend you to their backs," not only drips with irony (he will attempt to kill them on their ride), but it suggests that Macbeth has reduced Banquo to below the status of an animal. He will kill his best friend but not his horse.
There's also much time imagery in the play: clocks, bells, knocks, "When shall we three meet again?" Here, Macbeth says, "Let every man be master of his time." In timing his murder, Macbeth needs to know what time they ride so he can have his assassins waiting. In the context of the play, Macbeth races against time: he must Duncan in time; kill Banquo and Fleance in time; meet with the Witches in time. Time obviously will catch up to Macbeth in the forms of his wife's death and the arrival of the English army.
Verbal irony is found when Macbeth says, "Attend those men our pleasure?", speaking of the murderers. He uses "Sweeter welcome" as verbal irony for death, and he accuses the King's sons of "filling their hearers with a strange invention" when he, of course, is doing just that by accusing them.