The Roman History · Amm. 26.4.1

But in Bithynia, Valentinian, as we have already mentioned, having been declared emperor, having fixed the next day but one for beginning his march, assembled his chief officers, and, as if the course which he preferred was to follow their advice, inquired whom they recommended him to take for his colleague; and when no one made him any answer, Dagalaiphus, who at that time was commander of the cavalry, boldly answered "If, O excellent emperor, you love your own kindred, you have a brother; if you love the republic, then seek the fittest man to invest."

Ammianus Marcellinus · translated by C. D. Yonge · 1911 reprint
C. D. Yonge translation (preface dated February 1862); G. Bell and Sons, 1911; Project Gutenberg #28587
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