The first time the narrator's mother rebukes him is in regards to?

The first time the narrator's mother ever rebukes him is directly related to the language he uses.

From the text:

On one such afternoon one of the black boys turned suddenly on his tormentors and hurled a slate; it struck one of the white boys in the mouth, cutting a slight gash in his lip. At sight of the blood the boy who had thrown the slate ran, and his companions quickly followed. We ran after them pelting them with stones until they separated in several directions. I was very much wrought up over the affair, and went home and told my mother how one of the "niggers" had struck a boy with a slate. I shall never forget how she turned on me. "Don't you ever use that word again," she said, "and don't you ever bother the colored children at school. You ought to be ashamed of yourself." I did hang my head in shame, not because she had convinced me that I had done wrong, but because I was hurt by the first sharp word she had ever given me.