Source of funds is transactional: which account did this wire come from and what generated it. Source of wealth is biographical: how did this person accumulate assets at all. Regulators expect firms to distinguish them, because a clean bank transfer says nothing about the underlying wealth.
Acceptable evidence includes payslips, tax returns, sale contracts, audited company accounts, inheritance documents and trading statements. Screenshots and unverified declarations rarely suffice for enhanced-due-diligence.
Traders funding accounts from crypto gains meet this often, because on-chain history is not self-evidently legitimate and firms will ask for exchange records tracing the original fiat.
Related: enhanced-due-diligence, customer-due-diligence, politically-exposed-person, travel-rule, anti-money-laundering