In Exodus 17, which rock did God tell Moses to strike for water?

Answer: The rock at Horeb

God said He would stand before Moses by the rock at Horeb; Moses struck it and water came out for the people.

“Behold, I will stand there before you by the rock at Horeb. And when you strike the rock, water will come out of it for the people to drink.” So Moses did this in the sight of the elders of Israel.”— Exodus 17:6 (BSB)