He that complies against his will.
Is of his own opinion still.

Canto III, line 547. Sometimes misreported as "is convinced" instead of "complies"; reported in Paul F. Boller, Jr., and John George, They Never Said It: A Book of Fake Quotes, Misquotes, & Misleading Attributions (1989), p. 11 - Hudibras Part III (1678)