There's this possible quotation: "If we give in on this issue [slavery], there will be trouble a hundred years hence; posterity will never forgive us." Samuel Adams to Benjamin Franklin in July 1776 at the Continental Congress.

... but I can find that only according to Sherman Edwards and Peter Stone in an essay of how their musical 1776 differed from reality. (I have a copy of the book with their libretto, and I believe the printed text matches this Web page.) Can anyone find such a quotation elsewhere?

And in any event, it's not a prediction of civil war per se.