I'm fairly sure it's somewhere in the appendicies at the end of The Lord of the Rings. There's a considerable section devoted to the history of the Rohirrim.

If it's not in there, then it's somewhere in The Two Towers, probably in one of those scenes where the group travels to a new location so Aragorn or Legolas decides to educate and entertain the others by reciting all 46 songs, 23 poems and 9 epic tales he's ever heard about the place.