The amount of financial resources required differs depending on the destination country and length of stay. Sufficient assets can be proven with a bank statement. If you do not have sufficient funds yourself, you can provide proof by means of a formal invitation letter. The invitation must be issued by a resident of the destination country. The corresponding document is to be completed and signed, including financial proof on the part of the inviting person (bank statement / the 3 last salary statements / for a self-employed person, an income tax certificate from a tax adviser) with the visa application.