Albert Barnes Commentary Exodus 17:12

"But Moses` hands were heavy; and they took a stone, and put it under him, and he sat thereon; 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 down of the sun." — Exodus 17:12 (ASV)

Until the going down of the sun - The length of this first great battle indicates the strength and obstinacy of the assailants. It was no mere raid of Bedouins, but a deliberate attack of the Amalekites, who had probably been thoroughly trained in warfare by their struggles with Egypt.