John Wesley on Exodus 17:11

And when Moses held vp his hand in prayer (so the Chaldee explains it) Israel prevailed, but when he let down his hand from prayer, Amalek prevailed - To convince Israel that the hand of Moses (with whom they had just now been chiding) contributed more to their safety than their own hands; the success rises and falls, as Moses lifts up or lets down his hand. The church's cause is ordinarily more or less successful, according as the church's friends are more or less fervent in prayer.