Are you sure that's what they want you to prove?

If so, simply providing your public address would be enough, since all incoming and outgoing transactions are recorded on the blockchain and available for anyone to see.

However, what I believe they want you to prove is how you acquired all those bitcoins, whether you have a job, whether you earned them in an old ICO trade or bet, etc. But if your source of income is an old bitcoin HODL, then the blockchain records will indeed be your greatest ally as proof.

Good luck, and if you can provide us with more details than what was requested, it might be easier for us to help you.