عَيْنًا (a spring): A spring bursting from a high place in the Paradise. We know that a spring water comes from below, but this spring comes from above.
بِهَا (by): We do not drink from it but (by) it. It is a very strange image that we cannot imagine. It is very different from what we see in life, so do not think that the wine of the last Day is the same one of this life.