1 Samuel 18:3

Then Jonathan and David made a covenant
A covenant of friendship; entered into a solemn agreement to keep up and maintain a cordial respect to each other, and to support each other's interest both in life and after death, whoever was the survivor; and in consequence of this David had a friend at court, when Saul fell out with him, and who pleaded his cause, and discovered his father's plots, and was the means of preserving David's life:

because he loved him as his own soul;
so that this covenant was not founded in mere words, but in sincere and cordial affection, and was lasting and inviolable.