Psalms 136:12

With a strong hand, and with a stretched out arm: for his mercy endureth for ever.

The strong hand and the stretched-out arm describe God's deliverance of Israel from Egypt. That mighty rescue displayed His mercy as enduring forever. The returned exiles took up this praise to honor God as the same mighty Leader who had acted for them. They celebrated His merciful care as guiding and redeeming His people throughout their whole history.