Exodus 17:12

10 Joshua did as Moses had instructed him and fought against the Amalekites, while Moses, Aaron, and Hur went up to the top of the hill.

11 As long as Moses held up his hands, Israel prevailed; but when he lowered them, Amalek prevailed.

When Moses’ hands grew heavy, they took a stone and put it under him, and he sat on it. Then Aaron and Hur held his hands up, one on each side, so that his hands remained steady until the sun went down.

13 So Joshua overwhelmed Amalek and his army with the sword.

14 Then the LORD said to Moses, “Write this on a scroll as a reminder and recite it to Joshua, because I will utterly blot out the memory of Amalek from under heaven.”

Do read the surrounding verses too, because the context often says more than a single verse.

Compare

BSB
When Moses’ hands grew heavy, they took a stone and put it under him, and he sat on it. Then Aaron and Hur held his hands up, one on each side, so that his hands remained steady until the sun went down.
KJV
But Moses’ hands were heavy; and they took a stone, and put it under him, and he sat thereon; and Aaron and Hur stayed up his hands, the one on the one side, and the other on the other side; and his hands were steady until the going down of the sun.
WEB
But Moses’ hands were heavy; so they took a stone, and put it under him, and he sat on it. Aaron and Hur held up his hands, the one on the one side, and the other on the other side. His hands were steady until sunset.
ASV
But Moses` hands were heavy; and they took a stone, and put it under him, and he sat thereon; and Aaron and Hur stayed up his hands, the one on the one side, and the other on the other side; And his hands were steady until the going down of the sun.
DARBY
And Moses' hands were heavy; then they took a stone, and put [it] under him, and he sat on it; and Aaron and Hur supported his hands, one on this side, and one on that side; and his hands were steady until the going down of the sun.
BLB
And the hands of Moses [became] heavy, and they took a stone and put [it] under him, and he sat on it. And Aaron and Hur supported his hands—one from this, and one from that—and his hands were steady until the going down of the sun.
ÅB
Men da Moses' hender ble tunge, tok de en stein og la under ham, og han satte seg på den; og Aron og Hur støttet hans hender, den ene på den ene side og den andre på den andre side; så holdt hans hender seg støe helt til solen gikk ned.
OTB
Moses' hender ble trette, så de plasserte en stein under ham, og han satte seg på den. Aron og Hur støttet hendene hans, én på hver side, så hendene hans holdt seg støe til solnedgang.
B1930
Men da Moses' hender blev tunge, tok de en sten og la under ham, og han satte sig på den; og Aron og Hur støttet hans hender, den ene på den ene side og den andre på den andre side; så holdt hans hender sig støe helt til solen gikk ned.

Study notes (Tyndale)

Original text and word study

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