First stab was delivered, according to Plutarch, by Gaius Servilius Casca while his brother Publius was holding Caesar. Then the rest of conspirators started swarming Caesar and stabbing him. Main sources cite different last words: Suetonius mentions as a hearsay that Caesar said “You too, child?” in Greek. Plutarch wrote that Caesar didn’t say anything at this point but covered his face seeing Brutus among murderers, and his last words were “Casca, you villain, what are you doing?”, in Latin at Publius holding him so before the first stab.

So the famous “Et tu Brute” is Shakespeare joining both versions.