It's literally in Article 2 Section 1 of the Constitution:

"Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress: but no Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector."

The state legislatures are charged with putting forward electors in whatever manner they choose. There's nothing in the Constitution that says a state has to choose electors with a popular vote. In the first presidential election of 1789, only 6 of the 13 original states chose electors by popular vote. https://en.wikipedia.org/wiki/United_States_presidential_election#History

It's literally in Article 2 Section 1 of the Constitution:

"Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress: but no Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector."

The state legislatures are charged with putting forward electors in whatever manner they choose. There's nothing in the Constitution that says the state has to choose electors with a popular vote. During the first presidential election in 1789, only 6 of the 13 original states chose electors by popular vote. https://en.wikipedia.org/wiki/United_States_presidential_election#History