Targum Jonathan on Exodus
And the hands of Mosheh were heavy, because the conflict was prolonged till the morrow, and the deliverance of Israel was not prepared on that day; and he could not hold them up in prayer; on which account he would have afflicted his soul. And they took a stone, and placed it under him, and he sat upon it; and Aharon and Hur supported his hand, this the one, and that the other; and his hands were outstretched with firmness, (or, fidelity,) in prayer and fasting, until the going down of the sun.