Evan, it's not a poem. It's a famous quotation from Thoreau's book "Walden".
You can read some information about it here:
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"If a man does not keep pace with his companions, perhaps it is because he hears a different drummer. Let him step to the music which he hears, however measured or far away."
----- Henry David Thoreau, Walden, Conclusion, 1854
Les