The verse says Moses' hands were "steadfast" during the battle against Amalek (Exodus 17:12). The Mekhilta reads that single word as a double testimony — each of Moses' two hands testified to something different.
One hand was steadfast because Moses had never taken anything from the people of Israel. Throughout decades of leadership — guiding them out of Egypt, adjudicating their disputes, mediating between them and God — he had never enriched himself at their expense. Not a single donkey, not a single gift. His hand was clean, and that cleanness gave it strength.
The other hand was steadfast because of what Moses now said in prayer: "Lord of the universe, by my hand You took Israel out of Egypt. By my hand You split the sea for them. By my hand You performed for them miracles and acts of might. So by my hand, perform miracles and acts of might for them at this time as well."
Moses was not boasting. He was making a legal argument before the heavenly court. If God had already established a pattern — using Moses' hands as the instrument of deliverance — then consistency demanded those same hands deliver Israel again. The past miracles became precedent. The hands that parted the Red Sea were now raised over the battlefield, and Moses was essentially saying: You chose these hands. Do not abandon them now.