Verse Ex17-12c discussing Moses' prayers for t he Jewish people during the war with Amalayk states But Moses’ hands were heavy; and they took a stone, and put it under him, and he sat on it; and Aaron and Hur stayed up his hands, the one on the one side, and the other on the other side; and his hands were steady until the going do

The rules of symbolism, listed in the above article, require symbolic interpretation when a verse description is anomolous. Here Moses supporting his hand with a stone versus a pillow is anomolous.

The rules of symbolism, listed in the above article, state that when symbolic interpretation is required, we interpret items by function, form, linguistic association, and Biblical assocaition. In this case Rashi uses the form of resting on a stone: Resting on a stone is painful. Moses abstained from resting on a pillow to symbolically affirm that he empathized with the suffering of the Jewish people. This empathy was a source of strength to them and encouraged their fighting. Even though Moses was an angel and did not need war he emphasized his empathy with the plight of the Jewish people.