출애굽기 17:13의 Musar

Other Rabbis in that Midrash add that the "ox" is a reference to Joseph, that the "donkey" is a reference to Issachar who is described in terms of a patient and hard working donkey in Jacob's blessings in Genesis 49,14. Joshua, a direct descendant of Joseph, would defeat Amalek in battle. The sons of Issachar, steeped in Torah study, know how G–d rules His universe. This is why the emissaries they sent to attend the crowning of King David are referred to as יודעי בינה לעתים, (Chronicles I 12, 33) "who know how to interpret the signs of the times." At that point in history they knew what was required from the people of Israel. The עבד and שפחה may allude to David and Abigail. A Biblical allusion is also cited for this. Thus far the Midrash.