Each state needs to pass a law requiring ALL political donations to candidates for political representation of their state and their citizens must originate from resident citizens of their state.
Then, during primaries, candidates for the office of President of the United States would provide audited documentation to confirm that donations to their political campaign originated from U.S. citizens. No law is really needed for that purpose. President Trump could set the standard for the 2020 election. Candidates who did not provide audited documentation would not survive the primaries.