R. Eliezer says: What is the intent of "and Israel will grow strong" or "and Amalek will grow strong"? — Whenever Moses would raise his hands heavenward, Israel strengthened themselves in words of Torah, which were destined to be given by his hands. And whenever he lowered his hands, Israel weakened in words of Torah, which were destined to be given by his hands.
When Moses Raised His Hands Toward Heaven, Israel Grew Strong
Mekhilta Tractate Amalek 1:28