“If you can't say something good about someone, sit right here by me.”
As quoted in TIME (9 December 1966) - photograph of her with a pillow bearing the quotation http://www.usatoday.com/life/books/reviews/2007-12-12-roosevelt_N.htm
Variants:
If you haven't anything nice to say about anybody, come sit next to me.
If you haven't something good to say about someone sit right here by me.