I warn everyone who hears the words of the prophecy of this scroll: If any one of you adds anything to them, God will add to you the plagues described in this scroll. (NIV)

I [personally solemnly] warn everyone who listens to the statements of the prophecy [the predictions and the consolations and admonitions pertaining to them] in this book: If anyone shall add anything to them, God will add and lay upon him the plagues (the afflictions and the calamities) that are recorded and described in this book. (AMP)

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: (KJV)