As a component of discourse, asking for a source during an online discussion implies that such a source is not trivial to find, and thus calls into question the assertion for which a source is being asked for. Prepending your inquiry with "Really?" also contributed to that note of incredulity.

Particularly in a case where a source or citation is extremely easy to find, asking for one comes off as less of an honest inquiry and more of a rhetorical device designed to discredit the person making the assertion, regardless of intent.

I think a modicum of effort instead of wasting everyone's time with what is a well known fact would be polite.

It can disrupt conversation to ask for a source when you could find it yourself in less time than it takes to ask. This is especially true when phrased in a curt manner that implies you already doubt the claim.