Strabo Geography 16.1.28

[28] The Euphrates and its eastern banks are the boundaries of the Parthian empire. The Romans and the chiefs of the Arabian tribes occupy the parts on this side the Euphrates as far as Babylonia. Some of the chiefs attach themselves in preference to the Parthians, others to the Romans, to whom they adjoin. The Scenitae nomades, who live near the river, are less friendly to the Romans than those tribes who are situated at a distance near Arabia Felix. The Parthians were once solicitous of conciliating the friendship of the Romans, but having repulsed Crassus, ¹ who began the war with them, they suffered reprisals, when they themselves commenced hostilities, and sent Pacorus into Asia. ² But Antony, following the advice of the Armenian, ³ was betrayed, and was unsuccessful (against them). Phraates, his ⁴ successor, was so anxious to obtain the friendship of Augustus Caesar, that he even sent the trophies, which the Parthians had set up as memorials of the defeat of the Romans. He also invited Titius to a conference, who was at that time prefect of Syria, and delivered into his hands, as hostages, four of his legitimate sons, Seraspadanes, Rhodaspes, Phraates, and Bonones, with two of their wives and four of their sons; for he was apprehensive of conspiracy and attempts on his life. ⁵ He knew that no one could prevail against him, unless he was opposed by one of the Arsacian family, to which race the Parthians were strongly attached. He therefore removed the sons out of his way, with a view of annihilating the hopes of the disaffected.

The surviving sons, who live at Rome, are entertained as princes at the public expense. The other kings (his successors) have continued to send ambassadors (to Rome), and to hold conferences (with the Roman prefects).