PETER: To die will be an awfully big adventure.

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

Peter Pan, Act 3 (1904, pub. 1928)
    (Source)

This was added to the play in 1905, at the end of Act 3:

(The waters are lapping over the rock now, and PETER knows that it will soon be submerged. Pale rays of light mingle with the moving clouds, and from the coral grottoes is to be heard a sound, at once the most musical and the most melancholy in the Never Land, the mermaids calling to the moon to rise. PETER is afraid at last, and a tremor runs through him, like a shudder passing over the lagoon; but on the lagoon one shudder follows another till there are hundreds of them, and he feels just the one.)
PETER (with a drum beating in his breast as if he were a real boy at last): To die will be an awfully big adventure.

In Barrie's novelization, Peter and Wendy, ch. 8 "The Mermaids' Lagoon" (1911), this is rendered:

The rock was very small now; soon it would be submerged. Pale rays of light tiptoed across the waters; and by and by there was to be heard a sound at once the most musical and the most melancholy in the world: the mermaids calling to the moon.
Peter was not quite like other boys; but he was afraid at last. A tremor ran through him, like a shudder passing over the sea; but on the sea one shudder follows another till there are hundreds of them, and Peter felt just the one. Next moment he was standing erect on the rock again, with that smile on his face and a drum beating within him. It was saying, “To die will be an awfully big adventure.”

Sometimes given as "To die would be an awfully great adventure," "To die will be a great adventure," or "To die would be a great adventure."