Caesar echoes these words just before dieing in the play, "Julius Caesar" by William Shakespeare. It translates to, "You too Brutus?" Brutus is the character in the tragedy that betrays Caesar. The line is frequently referenced whenever someone is betrayed by someone they thought was their friend.
Although not in the text of the play, Caesar most likely would repeat this line in Act V of the play. Three characters commit suicide in this Act, Cassius, Titinius, and Brutus. Before the great battle, Brutus sees Caesar's ghost. Brutus asks Caesar what he is, to which he replies, "Thy evil spirit Brutus." this foreshadows Brutus' assisted suicide (by his servants); Caesar is trying to avenge his own death, and killing Brutus would do that.
One can picture Caesar's ghost entering the body of one of Brutus' servants as Brutus impales himself. Caesar has the beautiful wings akin to that of the archangels, spread wide displaying the full glory of Elysium. Caesar screams no longer a question as he stabs Brutus, but a warcry, "Et tu Brute!"