Our son just travelled to Massachusetts and he was tested 3 days before. He had proof that is was negative. No one in Massachusetts ever requested proof.
All visitors entering Massachusetts, including returning residents, who do not meet an exemption, are required to:
Complete the Massachusetts Travel Form prior to arrival, unless you are visiting from a lower-risk state designated by the Department of Public Health.
Quarantine for 14 days or produce a negative COVID-19 test result that has been administered up to 72-hours prior to your arrival in Massachusetts.
If your COVID-19 test result has not been received prior to arrival, visitors, and residents must quarantine until they receive a negative test result.
Failure to comply may result in a $500 fine per day.