Who says it is a sin To Kill a Mockingbird in the book To Kill a Mockingbird?

Atticus mentioned this sentence in the 1st part of the book. He was teaching Jem to shoot with an air gun. After Jem learned how to use the gun, he tried to train by shooting birds. It was his initial decision and he requested for Atticus' permission to do so.

At this moment, Atticus said that Jem could shoot all the bluejays that he wanted, but reminded him not to shoot any mockingbirds. He said that "it was a sin To Kill a Mockingbird," and his reason was that "Mockingbirds do not a thing but sing their hearts out for us, so it a sin to kill a mockingbird."

Atticus