The first two kings, Saul and David, feature prominently in the first book of Samuel.
The First Book of Samuel is the ninth book in the Bible. It tells us about the prophet Samuel and the transition of Israel from the rule of Judges to that of Kings.
I Samuel 18:3-4
"Then Jonathan and David made a covenant, because he loved him as his own soul. And Jonathan stripped himself of the robe that was upon him, and gave it to David, and his garments, even to his sword, and to his bow, and to his girdle"