Much if not all, the story of King Arthur is fictional. This battle with Lucius Tiberius, a supposed and otherwise unknown Roman Procurator**, first appeared in Geoffrey of Monmouth's Historia Regum Britanniae,  written c1136. However, with few other speeches from this period, it not only tells us what was expected from such a speech, but it may well be based on actual speeches delivered in the field.

(**there are passages in Geoffrey that give him the title Emperor).

Arthur's exhortation to his soldiers.

After he had thus placed them all in their stations, he made the following speech to his soldiers:--

"My brave countrymen, who have made Britain the mistress of thirty kingdoms, I congratulate you upon your late noble exploit, which to me is a proof that your valour is so far from being impaired, that it is rather increased. Though you have been five years without exercise, wherein the softening pleasures of an easy life had a greater share of your time than the use of arms; yet all this has not made you degenerate from your natural bravery, which you have shown in forcing the Romans to flee. The pride of their leaders has animated them to attempt the invasion of your liberties. They have tried you in battle, with numbers superior to yours, and have not been able to stand before you; but have basely withdrawn themselves into that city, from which they are now ready to march out, and to pass through this valley in their way to Augustodunum; so that you may have an opportunity of falling upon them unawares like a flock of sheep. Certainly they expected to find in you the cowardice of the Eastern nations, when they thought to make your country tributary, and you their slaves. What, have they never heard of your wars, with the Dacians, Norwegians, and princes of the Gauls, whom you reduced under my power, and freed from their shameful yoke? We, then, that have had success in a greater war, need not doubt of it in a less, if we do but endeavour with the same spirit to vanquish these poltroons. You shall want no rewards of honour, if as faithful soldiers you do but strictly obey my commands. For as soon as we have routed them, we will march straight to Rome, and take it; and then all the gold, silver, palaces, towers, towns, cities, and other riches of the vanquished shall be yours."

He had hardly done speaking before they all with one voice declared, that they were ready to suffer death, rather than quit the field while he had life.  

[Geoffrey of Monmouth, book 6, Chap.VII]