Ammianus Marcellinus Res Gestae 26.8.3

[3] Finally, worn out by scarcity of supplies and the very obstinate resistance of the defenders, he was already pre- paring to depart, when those who had meanwhile been blockaded at Nicaea suddenly opened the gates and rushed out, and after slaying a great part of the besiegers, headed by their bold leader, Rumitalca, hastened eagerly on with the purpose of surrounding Valens from the rear; for he had not left the suburb of Chalcedon. And they would have been successful, if the emperor had not from an earlier rumour learned of the danger that threatened him, and by a hasty retreat by way of the Sunonian lake ¹ and the many windings of the river Gallus ² outwitted the enemy, who were close upon his heels in vain pursuit. And by this mischance Bithynia also fell into the power of Procopius.