I think it is better to post than not so long as you can provide some substance (not just a one-liner). You should acknowledge the lack of sources, and if you intend to add them within a reasonable amount of time (say a day) you should say that. If you're planning to add them "when you get around to it", which could be a while, then it is IMO better not to make the offer, because your saying so may cause others to decide they don't have to.
The problem with just posting a one-liner is that the answer probably needs more than just a citation; it needs that and a summary, because few questions are cut-and-dried (especially ones that make it to this site). By posting a too-terse answer and inviting others to provide the source, you give the impression (presumably unintended) of "staking out" the answer; if somebody else posts a sourced answer saying the same thing, it could lead to some minor bad feelings. (I know we shouldn't worry about reputation, but people do notice it and it's a key feature of SE.) If you provide a good answer and perhaps even an idea of where you might have heard it, then someone filling in the citation for you is -- comparatively speaking -- not doing a lot of the work. It's the same as when soemone edits another's answer to fix typos or clear up ambiguities or formatting or the like; it's a small thing that we all happily do to make the site better, and nobody worries about someobody else "profitting" from his work.
So if all you can post is something terse that will need to be fleshed out substantially, IMO it's better to post that as a comment. That way you provide what information you have but you leave it clear that the question is awaiting a fuller answer. If nobody has improved on that by the time you return with a source, you can post an answer then.