All right found it.
"Interesting that you refrain from calling me your Master."
Vader inclined his head to Sidious. "I meant nothing by it, my Master. "
Sidious sneered. "Perhaps you wish you could strike me down, is that it?"
"No, Master."
"What stops you from doing so? Obi-Wan was once your Master, and you were certainly prepared to kill him. Even if you failed."
Vader clenched his right hand. "Obi-Wan did not understand the power of the dark side."
"And you do?"
"No, Master. Not yet. Not fully."
"And that's why you don't try to strike me down? Because I possess powers you lack?" Sidious lifted his arms, hands deployed like claws, as if to summon and hurl Sith lightning. "Because you know that I could easily overwhelm the delicate electrical systems of your suit."
Vader stood his ground. "I don't fear death, Master."
Sidious grinned maliciously. "Then why go on living, my young apprentice?"
Vader looked down at him. "To learn to become more powerful."
Sidious lowered his hands. "Then I ask you one final time, Lord Vader. Why not strike me down?"
"Because you are my path to power, Master," Vader said. "Because I need you."
Sidious narrowed his eyes and nodded. "Just like I needed my Master-for a time."
"Yes, Master," Vader said finally. "For a time."
"Good. Very good." Sidious smiled in satisfaction. "And now you are ready to release your anger."
I really wouldn't take that too literally, it seems more like an exageration on Sidious's part.