Deuteronomy 9:21

And I took your sin, the calf which ye had made, and burnt it with fire, and stamped it, and ground it very small, even until it was as small as dust: and I cast the dust thereof into the brook that descended out of the mount.

I took your sin - i:e., the fruit of your sin. I cast the dust thereof into the brook that descended out of the mount. It is too seldom borne in mind that though the Israelites were supplied with water from this rock when they were stationed at Rephidim (Wady Feiran), there is nothing in the Scripture narrative which should lead us to suppose that the rock was in the immediate neighbourhood of that place (see the notes at Exodus 17:5).

The water of this smitten rock was probably the brook that descended from the mount. The water may have flowed to the distance of many miles from the rock, as the winter torrents do now through the wadies of Arabia Petrea (Psalms 78:15). And the rock may have been smitten at such a height, and at a spot bearing such a relation to the Sinaitic valleys, as to furnish in this way supplies of water to the Israelites during the journey from Horeb by the way of mount Seir and Kadesh-barnea (Deuteronomy 1:1). On this supposition, new light is perhaps cast on the figurative language of the apostle, when he speaks of 'the rock following' the Israelites (1 Corinthians 10:4) (Wilson's 'Lands of the Bible').