There has never been a case where a president resisted voluntary compliance when threatened with a subpoena. Clinton was subpoenaed, but it was withdrawn when he agreed to testify voluntarily. The ultimate problem is that the Constitution contains language that is vague on the subject. So, although there is controversy, the prevailing opinion is that the Constitution implies that a President can only be tried legally after he leaves office. Here is the relevant passage:
Judgment in Cases of Impeachment shall not extend further than to removal from office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States: but the Party convicted shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law.
This language appears to lay out a sequence of events: first impeachment and conviction by Congress, second is the legal trial. See this opinion piece from Bloomberg:
A Sitting President Can't Be Prosecuted.
Richard Nixon could have been tried for the crimes he committed while in office, and that is why President Ford pardoned him. At the time, Ford claimed that it was to spare the country from the embarrassment and distraction of a former President being tried in criminal court, but it was likely part of a deal with Nixon to spare him (and the Republican Party) that embarrassment.