Res Gestae
Ammianus Marcellinus
Ammianus Marcellinus. Ammianus Marcellinus, with an English translation, Vols. I-III. Rolfe, John C., translator. Cambridge, MA; London: Harvard University Press; W. Heinemann, 1935-1940 (printing).
And they run over very great distances, pursuing others or themselves turning their backs,
v1.p.371
being mounted on swift and obedient horses and leading one, or sometimes even two, to the end that an exchange may keep up the strength of their mounts and that their freshness may be renewed by alternate periods of rest.