The first thing you have to do is to access your bnc10 account from the website because you can not get proof of transfer from the App.
There are different options:
Download the transaction history
From the home page (the site that appears when you log in from the website), at the beginning of the “Your account” section, you will see a printer icon. Click this button to download the transaction history.
You can download the transactions of a period of time or look for a concrete date by clicking the button that has a lens icon (next to the printer one).
Download the proof of transfer
From the home page, look for a specific transaction and select it by clicking at the arrow on the right side.
When the new screen appears, you will see, up on the right side, a button that says “pdf“. Click on it to download the proof of transfer.