The poem that named reindeer is "'Twas the Night Before Christmas" by Clement Clarke Moore.
While the poem doesn't actually name the reindeer, it does describe them as "eight tiny reindeer," giving us the following list:
* Dasher
* Dancer
* Prancer
* Vixen
* Comet
* Cupid
* Donner
* Blitzen
The names themselves are not explicitly stated in the poem, but they have become so commonly associated with Santa's reindeer that they are now considered canonical.