Variantly: "as a stone": because their hearts were hard as stone. But Your goodness and Your many lovingkindnesses and Your mercies are upon us, and Your right hand is stretched out to all who enter the world, "Your right hand," being written twice (viz. 15:6). (Psalms 44:4) "… but Your right hand, Your arm, and the light of Your countenance — for You favored them," and (Isaiah 45:23) "By Myself I have sworn. From My mouth has gone forth righteousness, a word that will not turn back."
As a Stone Because Their Hearts Were Hard as Stone
Mekhilta Tractate Shirah 5:7