How do I provide proof of a transaction for tax purposes? How do I provide proof of a transactions for taxation purposes?
I suppose that you want to (legally, not scientifically (Big difference!)) prove that you issued a certain transaction.

First off: State the transaction ID so it can easily be looked up.

You can prove that you are in possession of the private key which issued the transaction by signing a message using that private key. How easy this is depends on your wallet and if your wallet doesn't support it, it depends on your operating system and the software available for it.

With Electrum, it's super easy:

Just right-click the address you sent the money from and choose Sign/verify message.



This window opens:



Enter a message which says that you (state your full name) issued the transaction:



Click the Sign button. You will be prompted for your password. After entering it, the signature will be printed in the bottom text area:



State this signature along the message you entered (Every character needs to be exactly the same!) and the transaction ID.