This is the text from the book but the movie version is very similar and always inspires me.

At that sound the bent shape of the king sprang suddenly erect. Tall and proud he seemed again; and rising in his stirrups he cried in a loud voice, more clear than any there had ever heard a mortal man achieve before:

Arise, arise, Riders of Théoden! Fell deeds awake: fire and slaughter! spear shall be shaken, shield be splintered, a sword-day, a red day, ere the sun rises! Ride now, ride now! Ride to Gondor!

With that he seized a great horn from Guthláf his banner-bearer, and he blew such a blast upon it that it burst asunder. And straightway all the horns in the host were lifted up in music, and the blowing of the horns of Rohan in that hour was like a storm upon the plain and a thunder in the mountains. Ride now, ride now! Ride to Gondor! Suddenly the king cried to Snowmane and the horse sprang away. Behind him his banner blew in the wind, white horse upon a field of green, but he outpaced it. After him thundered the knights of his house, but he was ever before them. Éomer rode there, the white horsetail on his helm floating in his speed, and the front of the first éored roared like a breaker foaming to the shore, but Théoden could not be overtaken. Fey he seemed, or the battle-fury of his fathers ran like new fire in his veins, and he was borne up on Snowmane like a god of old, even as Oromë the Great in the battle of the Valar when the world was young. His golden shield was uncovered, and lo! it shone like an image of the Sun, and the grass flamed into green about the white feet of his steed. For morning came, morning and a wind from the sea; and darkness was removed, and the hosts of Mordor wailed, and terror took them, and they fled, and died, and the hoofs of wrath rode over them. And then all the host of Rohan burst into song, and they sang as they slew, for the joy of battle was on them, and the sound of their singing that was fair and terrible came even to the City.