For reference, I found an online copy of the book here, which is where I pulled the earlier quote. So as another poster pointed out, killing mockingbirds is referenced twice, and this is the one you mean:

Atticus sat looking at the floor for a long time. Finally he raised his head. "Scout," he said, "Mr. Ewell fell on his knife. Can you possibly understand?"

Atticus looked like he needed cheering up. I ran to him and hugged him and kissed him with all my might. "Yes sir, I understand," 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?"

But searching for that, I found one of my favorite sections, also near the end:

Neighbors bring food with death and flowers with sickness and little things in between. Boo was our neighbor. He gave us two soap dolls, a broken watch and chain, a pair of good-luck pennies, and our lives. But neighbors give in return. We never put back into the tree what we took out of it: we had given him nothing, and it made me sad.

I turned to go home. Street lights winked down the street all the way to town. I had never seen our neighborhood from this angle. There were Miss Maudie's, Miss Stephanie's—there was our house, I could see the porch swing—Miss Rachel's house was beyond us, plainly visible. I could even see Mrs. Dubose's.

I looked behind me. To the left of the brown door was a long shuttered window. I walked to it, stood in front of it, and turned around. In daylight, I thought, you could see to the postoffice corner.

Daylight… in my mind, the night faded. It was daytime and the neighborhood was busy. Miss Stephanie Crawford crossed the street to tell the latest to Miss Rachel. Miss Maudie bent over her azaleas. It was summertime, and two children scampered down the sidewalk toward a man approaching in the distance. The man waved, and the children raced each other to him.

It was still summertime, and the children came closer. A boy trudged down the sidewalk dragging a fishingpole behind him. A man stood waiting with his hands on his hips. Summertime, and his children played in the front yard with their friend, enacting a strange little drama of their own invention.

It was fall, and his children fought on the sidewalk in front of Mrs. Dubose's. The boy helped his sister to her feet, and they made their way home. Fall, and his children trotted to and fro around the corner, the day's woes and triumphs on their faces. They stopped at an oak tree, delighted, puzzled, apprehensive.

Winter, and his children shivered at the front gate, silhouetted against a blazing house. Winter, and a man walked into the street, dropped his glasses, and shot a dog.

Summer, and he watched his children's heart break. Autumn again, and Boo's children needed him.

Atticus was right. One time he said you never really know a man until you stand in his shoes and walk around in them. Just standing on the Radley porch was enough.