A recent statement (extracto bancario) from a bank account in your name showing you can cover the full requested stay. Spain measures this against the IPREM (Indicador Público de Renta de Efectos Múltiples): the rule for students is 100% IPREM per month of stay.
Download a PDF statement from your bank's app or web portal for the most recent month, in your name, with the bank's logo visible. If a parent or sponsor supports you, add a notarized declaration of financial support plus their bank statement showing the same amounts.