“Here is the piece. If you can't say fornicate can you say copulate or if not that can you say co-habit? If not that would have to say consummate I suppose. Use your own good taste and judgment.”

Source: I Am the Messenger

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.

1860s, Cooper Union speech (1860)
Context: But you will not abide the election of a Republican president! In that supposed event, you say, you will destroy the Union; and then, you say, the great crime of having destroyed it will be upon us! That is cool. A highwayman holds a pistol to my ear, and mutters through his teeth, "Stand and deliver, or I shall kill you, and then you will be a murderer!" To be sure, what the robber demanded of me — my money — was my own; and I had a clear right to keep it; but it was no more my own than my vote is my own; and the threat of death to me, to extort my money, and the threat of destruction to the Union, to extort my vote, can scarcely be distinguished in principle.

Source: The Bean Trees

Will and Mary in Ch. 33 : Marzipan
His Dark Materials, The Amber Spyglass (2000)
Context: They lay back, well fed and comfortable in the flower-scented night, and listened to Mary tell her story.
She began just before she first met Lyra, telling them about the work she was doing at the Dark Matter Research group, and the funding crisis. How much time she’d had to spend asking for money, and how little time there’d been left for research!
But Lyra’s coming had changed everything, and so quickly: within a matter of days she’d left her world altogether.
"I did as you told me," she said. "I made a program — that’s a set of instructions — to let the Shadows talk to me through the computer. They told me what to do. They said they were angels, and — well…"
"If you were a scientist," said Will, "I don’t suppose that was a good thing for them to say. You might not have believed in angels."
"Ah, but I knew about them. I used to be a nun, you see. I thought physics could be done to the glory of God, till I saw there wasn’t any God at all and that physics was more interesting anyway. The Christian religion is a very powerful and convincing mistake, that’s all."

Popular Quotes, Essentialism: The Disciplined Pursuit of Less, Twitter

Source: The Mike Wallace Interview (1958)