Their bank statement of past 4 months and a proof of annual income. If they own any other property like land, business then attach a proof of it. The more you show the better it is.
Only needs 4-month bank statement? I remember it requres at least one year bank statement.
And also, is there any requirement about how long the money needs to be saved in the bank? Because the majoriy amount of the money in my parents' account was just deposited into the account ( no longer than a month ). Will that be OK? Or do my parents need to provide the proof where they get this money?
If u provide a year, thats better, ckeck embassy website to be sure. Of course, they may question this transaction just a month before. So be prepared to explain it.