John Gill Commentary Psalms 114:8

"Who turned the rock into a pool of water, The flint into a fountain of waters." — Psalms 114:8 (ASV)

Which turned the rock into a standing water
Both at Rephidim and at Kadesh; which being smitten, streams of water flowed out like rivers, as if the rock itself was changed into water; and which came a constant and continual supply for the Israelites, for it is said to follow them; see (Exodus 17:6) (Numbers 20:11) (Psalms 78:15Psalms 78:16) (105:41) (1 Corinthians 10:4) .

The flint into a fountain of waters ;
referring to the same thing, the rocks were flinty ones. This was a type of Christ the Rock; who has an abiding fulness of grace in him; is the fountain of it, from where it flows in great abundance for the supply of his people's wants, while passing through this wilderness to Canaan's land.