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

[263] And when he had given them this answer, he sent the ambassadors away. And when he had prayed for victory, and had vowed to perform sacred offices, and if he came home in safety, to offer in sacrifice what living creature soever should first meet him, ¹ he joined battle with the enemy, and gained a great victory, and in his pursuit slew the enemies all along as far as the city of Minnith. He then passed over to the land of the Ammonites, and overthrew many of their cities, and took their prey, and freed his own people from that slavery which they had undergone for eighteen years. But as he came back, he fell into a calamity no way correspondent to the great actions he had done; for it was his daughter that came to meet him; she was also an only child and a virgin: upon this Jephtha heavily lamented the greatness of his affliction, and blamed his daughter for being so forward in meeting him, for he had vowed to sacrifice her to God. However, this action that was to befall her was not ungrateful to her, since she should die upon occasion of her father's victory, and the liberty of her fellow citizens: she only desired her father to give her leave, for two months, to bewail her youth with her fellow citizens; and then she agreed, that at the forementioned thee he might do with her according to his vow. Accordingly, when that time was over, he sacrificed his daughter as a burnt-offering, offering such an oblation as was neither conformable to the law nor acceptable to God, not weighing with himself what opinion the hearers would have of such a practice.