וַיּ֤וֹסֶף יְהֽוֹנָתָן֙ לְהַשְׁבִּ֣יעַ אֶת־דָּוִ֔ד בְּאַהֲבָת֖וֹ אֹת֑וֹ כִּֽי־אַהֲבַ֥ת נַפְשׁ֖וֹ אֲהֵבֽוֹ׃ (ס)
And Jonathan caused David to swear again, for the love that he had to him; for he loved him as he loved his own soul.
Read halakhic rulings (Jewish law) on I Samuel 20:17: and classical halakhic sources.