The metaphor "killing a mockingbird" comes up three or four times in the novel. The two most salient times when the phrase above is mentioned come from two sources.
The first mention of the mockingbird is found in chapter 10, page 7, of the 1988 edition. In it, Atticus first makes mention of the innocent mockingbird, and it is here when he first compares the bird to someone who is innocent and gets hurt for no reason.
Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird
This conversation is further supported by Miss Maudie, who agrees that Mockingbirds are beautiful and innocent creatures whose only duty is to bring music to our ears. Scout will later on come to terms with this analogy and will apply it to her own object of fixation, the enigmatic Boo.
The second time the mockingbird is openly mentioned is in chapter 30, where Scout talks about Boo Radley and realizes that he, too, is an innocent person that is consistently abused by others. Scout compares Boo to Tom Robinson and realizes that putting Boo through certain situations, just like it happened with Tom, would also be like killing a mockingbird. In the same edition of the book, this would be found on pages 66-68.
Atticus looked like he needed cheering up. [...]I reassured him. "Mr. Tate was right."
Atticus disengaged himself and looked at me. "What do you mean?"
"Well, it'd be sort of like shootin' a mockingbird, wouldn't it?"
Keep in mind that it is hard to keep up with page numbers given the many editions of the same book that are consistently released. Your best bet would be to opt for an electronic book (ebook) or an electronic publication (epub) and search within it to find page numbers, making sure that you address in your homework which edition of the book you are using.