The poem is Ulysses by Alfred Lord Tennyson. This is the quote:
Though much is taken, much abides; and though
We are not now that strength which in old days
Moved earth and heaven, that which we are, we are;
One equal temper of heroic hearts,
Made weak by time and fate, but strong in will
To strive, to seek, to find, and not to yield.
The full poem is here.