Answer: "I am a Jedi, like my father before me."
After Luke furiously slices off Vader's mechanical hand, he looks at his own that Vader took. He realizes that he is becoming too much like his father. Then he renounces the dark side, casting aside his lightsaber and proudly proclaiming to the Emperor, "Never. I'll never turn to the dark side. You've failed, your highness. I am a Jedi, like my father before me." So the third line in that quote is the final line he says before the attack. He says, "Father, please!" during the attack.