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

[175] Now while this war between the Hebrews and the Philistines was going on, Saul sent away David to his father Jesse, and contented himself with those three sons of his whom he had sent to his assistance, and to be partners in the dangers of the war: and at first David returned to feed his sheep and his flocks; but after no long time he came to the camp of the Hebrews, as sent by his father, to carry provisions to his brethren, and to know what they were doing. While Goliath came again, and challenged them, and reproached them, that they had no man of valor among them that durst come down to fight him; and as David was talking with his brethren about the business for which his father had sent him, he heard the Philistine reproaching and abusing the army, and had indignation at it, and said to his brethren, "I am ready to fight a single combat with this adversary." Whereupon Eliab, his eldest brother, reproved him, and said that he spoke too rashly and improperly for one of his age, and bid him go to his flocks, and to his father. So he was abashed at his brother's words, and went away, but still he spake to some of the soldiers that he was willing to fight with him that challenged them. And when they had informed Saul what was the resolution of the young man, the king sent for him to come to him: and when the king asked what he had to say, he replied, "O king, be not cast down, nor afraid, for I will depress the insolence of this adversary, and will go down and fight with him, and will bring him under me, as tall and as great as he is, till he shall be sufficiently laughed at, and thy army shall get great glory, when he shall be slain by one that is not yet of man's estate, neither fit for fighting, nor capable of being intrusted with the marshalling an army, or ordering a battle, but by one that looks like a child, and is really no elder in age than a child."