Biblical Antiquities (Pseudo-Philo) 61
1And after these things the Philistines came to fight against Israel. And David was returned to the wilderness to feed his sheep, and the Madianites came and would have taken his sheep, and he came down unto them and fought against them and slew of them 15,000 men. This is the first battle that David fought, being in the wilderness.
2And there came a man out of the camp of the Philistines by name Golia, and he looked upon Saul and upon Israel and said: Art not thou Saul which fleddest before me when I took the ark from you and slew your priests? And now that thou reignest, wilt thou come down unto me like a man and a king and fight against us? If not, I will come unto thee, and will cause thee to be taken captive, and thy people to serve our gods. And when Saul and Israel heard that, they feared greatly. And the Philistine said: According to the number of the days wherein Israel feasted when they received the law in the wilderness, even 40 days, I will reproach them, and after that I will fight with them.
4And Saul heard these words and sent and took him and said: What was the speech which thou spakest unto the people? And David said: Fear not, O king, for I will go and fight against the Philistine, and God will take away the hatred and reproach from Israel.
6And David went forth unto Golia and said unto him: Hear a word before thou diest. Were not the two women of whom thou and I were born sisters? and thy mother was Orpha and my mother was Ruth. And Orpha chose for herself the gods of the Philistines and went after them, but Ruth chose for herself the ways of the Most Mighty and walked in them. And now thou and thy brethren are born of Orpha, and as thou art arisen this day and come to lay Israel waste, behold, I also that am born of thy kindred am come to avenge my people. For thy three brethren also shall fall into my hands after thy death. And then shall ye say unto your mother: He that was born of thy sister hath not spared us.
8And David said unto him: Before thou diest, open thine eyes and behold thy slayer which hath killed thee. And the Philistine looked and saw the angel and said: Thou hast not killed me by thyself, but he that was with thee, whose form is not as the form of a man. And then David took his head from him.