From Antiquities Book 6:

8. But Jonathan heard these last words with indignation; and promised to do what he desired of him, and to inform him if his father’s answers implied any thing of a melancholy nature, and any enmity against him. And that he might the more firmly depend upon him, he took him out into the open field, into the pure air, and sware that he would neglect nothing that might tend to the preservation of David; and he said, “I appeal to that God, who, as thou seest, is diffused every where, and knoweth this intention of mine, before I explain it in words, as the witness of this my covenant with thee: that I will not leave off to make frequent trials of the purpose of my father, till I learn whether there be any lurking distemper in the secretest parts of his soul: and when I have learnt it, I will not conceal it from thee, but will discover it to thee; whether he be gently or peevishly disposed. For this God himself knows, that I pray he may always be with thee: for he is with thee now, and will not forsake thee; and will make thee superior to thine enemies; whether my father be one of them, or whether I my self be such. Do thou only remember what we now do: and if it fall out that I die, preserve my children alive; and requite what kindness thou hast now received, to them.”