I Samuel 18:3 Midrash

וַיִּכְרֹ֧ת יְהוֹנָתָ֛ן וְדָוִ֖ד בְּרִ֑ית בְּאַהֲבָת֥וֹ אֹת֖וֹ כְּנַפְשֽׁוֹ׃

Then Jonathan made a covenant with David, because he loved him as his own soul.

Read midrashic interpretation of I Samuel 18:3: and classical rabbinic exegesis.