The exact quote is:
"They are my old friends. I have heard you mention them with consideration these last twenty years at least."
It is spoken by Mr. Bennet in the first chapter of the novel to "his lady", for whom we have yet to be given a name.
Mr. Bennet calls his children silly and ignorant, and his lady asks how he can abuse his own children in such a way, and then mentions that he has "no compassion on [her] poor nerves." To this Mr. Bennet replies that to the contrary, he has become well acquainted with her nerves after, what we are to assume, the last twenty years of their marriage.