WENDY: Oh! Peter, when Captain Hook carried us away —

PETER: Who’s Captain Hook? Is it a story? Tell it me.

WENDY: (aghast) Do you mean to say you’ve even forgotten Captain Hook, and how you killed him and saved all our lives?

PETER: (fidgeting) I forget them after I kill them.

J. M. Barrie (1860-1937) Scottish novelist and dramatist [James Matthew Barrie]

When Wendy Grew Up — An Afterthought (1908, publ. 1957)
    (Source)

Most of When Wendy Grew Up was eventually folded into the evolving main play, Peter Pan, or The Boy Who Wouldn't Grow Up, Act 5 (first performed in 1904, eventually published in 1928), though these lines were not included.

In Barrie's 1911 novelization, Peter and Wendy, ch. 17 "When Wendy Grew Up," this is rendered:

She had looked forward to thrilling talks with him about old times, but new adventures had crowded the old ones from his mind.
“Who is Captain Hook?” he asked with interest when she spoke of the arch enemy.
“Don’t you remember,” she asked, amazed, “how you killed him and saved all our lives?”
“I forget them after I kill them,” he replied carelessly.