Where to get document proving sufficient income?

I am non-EU national, applying for Portugal working visa.In the list of required documents, there is one point which is not clear for me:

Documents proving that the applicant possesses sufficient income; bank statements (original, signed ans sealed by the bank and pay slips from the last 3 months);

As I haven't started working for my future employer, should I get it from my old employer? My current salary in my home country is not sufficient to live in Portugal

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