Flavius Josephus, Antiquities of the Jews
William Whiston, A.M., Ed.

[183] Now Herod, as soon as he had put Hyrcanus out of the way, made haste to Caesar; and because he could not have any hopes of kindness from him, on account of the friendship he had for Antony, he had a suspicion of Alexandra, lest she should take this opportunity to bring the multitude to a revolt, and introduce a sedition into the affairs of the kingdom; so he committed the care of every thing to his brother Pheroras, and placed his mother Cypros, and his sister [Salome], and the whole family at Masada, and gave him a charge, that if he should hear any sad news about him, he should take care of the government. But as to Mariamne his wife, because of the misunderstanding between her and his sister, and his sister's mother, which made it impossible for them to live together, he placed her at Alexandrium, with Alexandra her mother, and left his treasurer Joseph and Sohemus of Iturea to take care of that fortress. These two had been very faithful to him from the beginning, and were now left as a guard to the women. They also had it in charge, that if they should hear any mischief had befallen him, they should kill them both, and, as far as they were able, to preserve the kingdom for his sons, and for his brother Pheroras.