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 someone 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 somebody else "profiting" from his work.
So if all you can post is a terse pointer ("see Eruvin 13" or "Rambam Hilchot Teshuva covers this" or "(link)", etc), 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 real answer. Anybody -- the OP, another user, or you at a later time -- can then take that information and develop it into an answer, at which point the comment is obsolete.