Where did the term silence of the lambs come from?

In the movie, Clarice explains that she was traumatized as a child after waking up during the night to the sounds of lambs screaming as they were being slaughtered. Dr. Lector then comes to the conclusion that Clarice wants desperately to save Katherine Martin, the missing Senator's daughter, because she feels that by saving her, she'll be able to silence her memories of the lambs, whom she was unable to save. Dr. Lector refers back to this at the very end of the movie when he and Clarice speak on the phone, and he asks, "Well, Clarice, have the lambs stopped screaming?" He asks this because Clarice was successful in saving Katherine, and therefore should have succeeded in silencing those traumatic memories.