This is referred to several times in the Trump v. Vance decision, since it applies to your readings on civil liberties, here is more on it.
You'll find the phrase in the 6th Amendment, which regards trial procedures and specifically what rights criminal defendants have in a trial. In this case, there is a compulsory process for obtaining witnesses in one's favor.
In all criminal prosecutions, the accused shall enjoy the right...to have compulsory process for obtaining witnesses in his favor.
This is part of the due process of the law, which is the subject of the 4th - 8th amendments. The United States v Nixon applied this to the prosecution against Nixon, who was compelled to release evidence. That is the same basic argument used in Trump v Vance.
For more, click here.
