Well, he is a rich man getting to this question a kind, is a personal question. Like it's my account it's my decision if I want to share the transaction I made on the site or to the app that I used to buy.
Not a bad idea to ask for proof but still, we already saw the news. Is it not proof for you? that he really bought a billion worth of bitcoin there is so much news that spread the word on Elon Mush bought a billion worth of bitcoin.
I would also thought about it that maybe someone called from that site he used to make the transaction and called Elon Musk if he wants to share because personally for me, from a simple person like me and got rich or if I am a well know in the public I would not definitely show and spread the word of me buying a big amount of bitcoin if someone asks me if I could but it depends on the person.
Here is the thing I did and compare the technology today there is a chance if TESLA and Elon musk said what did they used to buy the transaction may be a lot of hackers would focus on the trading site or app they use and hack it, it's not impossible when you now know that you are one of the biggest holders of bitcoin.
Has anybody actually seen the Tesla purchase on the blockchain to prove it actually happened?..would be interesting to see it if it was a single transaction?