How do I prove to the authority that the money I got is from my winnings? Are there any documents to prove this?

There will be a letter provided by Magnum to certify that you are a winner.

Why Magnum

Exciting, Legit Fun: Magnum Corporation is the first company licensed to operate the exciting 4D Classic, including 4D Jackpot, 4D Jackpot Gold and Magnum Life.
Social Responsibility: Since inception in 1969, Magnum has been committed to a social policy of sustainable and impactful caring and sharing.
Full Payment Guaranteed: You have our guarantee that you’ll receive the full prize amount when you win any Magnum 4D Classic. No taxes, service charges or hidden costs!