Res Gestae

And the king, swearing with many an oath that he could sooner give up his life than his resolve, after receiving rewards returned to his kingdom with the retinue that he had brought with him; and after that he never dared to violate any of his promises, being bound to Constantius by many ties of gratitude, among which this was especially strong-that the emperor had given him to wife Olympias, daughter of Ablabius, a former praetorian prefect, and the betrothed of his brother Constans.

After the king had been sent off from Cappadocia, Constantius going by way of Melitena (a town of Lesser Armenia), Lacotena, and Samosata, crossed the Euphrates and came to Edessa. There he lingered for a long time, while he was waiting for the troops of soldiers that were assembling from