He is reciting part of the poem Ulysses by Tennyson, the same poem is mentioned in the Heinlein book I am currently re-reading "To sail beyond the sunset". Indeed the title itself is from the poem.
Oh, this clip also contains 2 of my most favorite quotes from Frasier which he may re-quoting but they are my all time favorite quotes anyway. Prizes for guessing which ones are those.
So, with that much ado, here's the clip.