It’s the first clause of the 12th Amendment to the United States Constitution, here’s the relevant text:

The Electors shall meet in their respective states, and vote by ballot for President and Vice-President, one of whom, at least, shall not be an inhabitant of the same state with themselves

You can read the full text on Wikipedia or on the official website of Congress.

What this would mean in practice, is that if Gavin Newsome were chosen as Kamala Harris’ running mate, is that many (if not the overwhelming majority) of Californian Presidential Electors would be prohibited from voting for Harris and Newsome, since the electors are generally chosen from local politicians and other locally influential individuals.

Harris needs every electoral vote she can get, and risking any of California’s many votes would be a significant threat to her election.

I hope that satisfies your need for citations, and please, correct me if I’ve interpreted the law wrong here.

Edit: grammar, and added my separate other comment to the end of the above citation.