a source field for each card

Often, when I am learning cards, I remember something from the subject of the source or I need more information (e. g. my own answer doesn't satisfied me) and at this point it would be great if I could look up where I originally have this card from (a web adress, page in a book, video etc.)
The source field doesn't have to show up which every answer, it is enough when it shows up on user's demand.

Of course i can just add the source in the answer-field - but this doesn't look nice and make the cards confusing :-)
And an extra field doesn't let me forgot to enter the source :P