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.

Don't make claims unless you're prepared to provide the source. It's not anyone's responsibility but your own should you put forth said claim.

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.