“If you bring these leaders to trial, it will condem the North, for by the Constitution, secession is not rebellion. His (Jefferson Davis’), capture was a mistake. His trial will be a greater one. We cannot convict him of treason. Secession is settled. Let it stay settled.” Chief Justice Salmon. P. Chase to Stanton See my reply 177 and 204.
Google it for yourself. The quote has been widely cited.
Cited but not sourced.