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

[224] And David went thence, and came to Jonathan, the son of Saul, and lamented to him what snares were laid for him by his father; and said, that though he had been guilty of no evil, nor had offended against him, yet he was very zealous to get him killed. Hereupon Jonathan exhorted him not to give credit to such his own suspicions, nor to the calumnies of those that raised those reports, if there were any that did so, but to depend on him, and take courage; for that his father had no such intention, since he would have acquainted him with that matter, and have taken his advice, had it been so, as he used to consult with him in common when he acted in other affairs. But David sware to him that so it was; and he desired him rather to believe him, and to provide for his safety, than to despise what he, with great sincerity, told him: that he would believe what he said, when he should either see him killed himself, or learn it upon inquiry from others: and that the reason why his father did not tell him of these things, was this, that he knew of the friendship and affection that he bore towards him.