Shmuel Alef 18:1-3

And it came to pass, when he had made an end of speaking unto Sha'ul, that the nefesh Yonatan was kashur (tied together) with the nefesh Dovid, and Yonatan loved him as his own nefesh. And Sha'ul drafted him that day, and would let him return no more to the bais aviv. Then Yonatan and Dovid made a brit (covenant), because he loved him as his own nefesh.