Lewis is quoting from Poem #1191 by Charles Kingsley, which reads,
My fairest child, I have no song to give you;
No lark could pipe to skies so dull and grey:
Yet, ere we part, one lesson I can leave you
For every day.
Be good, sweet maid, and let who will be clever;
Do noble things, not dream them, all day long:
And so make life, death, and that vast for-ever
One grand, sweet song.
Charles Kingsley was an Anglican priest who lived from 1819 to 1875.