Revelation 22:18:

For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book:

See also:

Objection: Wesley Said, “Whatever Is True Is Not New, and Whatever Is New Is Not True.”  Nobody Believed That Healing Was in the Atonement Until Recently, So It Is Not True.