Most of us can't prove the source of funds.
explain it to them in a term they can understand or familiar with
For instance, I have some income from a signature campaign. Try explaining to a bank clerk what a sig campaign is and proving that the funds come from there.
you can start by telling them your line of work can be classified into marketing
to be specific, you are doing an online advertising in a discussion forum
Try explaining to him that your income comes from claiming bitcoin forks, or that you claimed a faucet in 2012 and it happened to be worth 10k EUR now.
claiming forks ~= special dividents from your investments
small faucet amount turned into hefty value ~= cashing out long term investment, capital gain