Is hybridisation a source of variation among species or between species

It can be both. The swede plant, for example, is a distinct species from the closely related turnip and cabbage, and arose as a hybrid between both. In that instance, hybridization resulted in speciation.

Update
"Different species cannot reproduce with one another so it would have to be among."

The plant known as the swede thinks differently. It's disgracefully been able to breed with different species, and is a viable hybrid.

Different species cannot reproduce with one another so it would have to be among.

Since the offspring are usually sterile, probably not.