What did Elon Musk whisper about his job security at Trump's address to Congress?

Donald Trump during his joint address to Congress gave a shoutout to tech mogul Elon Musk and praised him for his work. Trump credited Musk with identifying "appalling waste" in federal spending, calling his work instrumental in streamlining government operations. During the speech, Musk whispered five-word comment to the man standing next to him.