Where is a mockingbird referenced in To Kill Mockingbird?

In To Kill Mockingbird by Harper Lee, several characters are compared to mockingbirds. This includes both Tom Robinson and Boo Radley. The title itself is a reference to a line in the book said by Atticus Finch, "Mockingbirds don't do one thing but make music for us to enjoy. They don't eat up people's gardens, don't nest in corncribs, they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird."