HERE it is on the CDC's website. If you are flying commercial you need a negative test within 3 days of getting on your flight. If you are driving, no test is required.
If you are flying a private aircraft there are different requirements, all explained on the CDC's website
On the
Dept of State's website HERE you can read all the rules for crossing the border on land. Only essential travel is allowed, but returning to the US by US citizen is considered essential.