Apollodorus Library 3.6.2

[2]
  But Amphiaraus, son of Oicles, being a seer and foreseeing that all who joined in the expedition except Adrastus were destined to perish, shrank from it himself and discouraged the rest. However, Polynices went to Iphis, son of Alector, and begged to know how Amphiaraus could be compelled to go to the war. He answered that it could be done if Eriphyle got the necklace. ¹ Now Amphiaraus had forbidden Eriphyle to accept gifts from Polynices; but Polynices gave her the necklace and begged her to persuade Amphiaraus to go to the war; for the decision lay with her, because once, when a difference arose between him and Adrastus, he had made it up with him and sworn to let Eriphyle decide any future dispute he might have with Adrastus. ² Accordingly, when war was to be made on Thebes, and the measure was advocated by Adrastus and opposed by Amphiaraus, Eriphyle accepted the necklace and persuaded him to march with Adrastus. Thus forced to go to the war, Amphiaraus laid his commands on his sons, that, when they were grown up, they should slay their mother and march against Thebes.