Taxing of bitcoin withdrawals

I bought some btc long back( say 0.1btc) in gate.io and stored in a cold wallet. Honestly I don’t even remember for which amount I bought it . Recently I sold it via an exchange and deposited the amount into my bank account . Now will I have to pay taxes on whole 6k amount in received into my bank account ? How will I prove that I bought that will 2k and now i have to be taxed only on the 4k which are my gains? There is no trace or details how much I bought it for and how much is my capital gain on it. Does any one have an idea?

Thanks in advance