In 20:13 it tells us that, "These are the waters of Meribah, where the children of Israel fought with the LORD, and He was sanctified in them."
But at the beginning of this incident in 20:2 it said they fought with Moshe. Did they fight with Moshe or with Hashem?
I would say, that is one and the same. Technically, they fought with Moshe, as the passuk first says. However, Moshe was a representative of Hashem to the people of Israel.
Picking a fight with Moshe is like picking a fight with Hashem.