We do have a "citation needed" post notice, but only moderators can add it to a post. The notification reads as:
This post does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
If you really think the citation needed notice should be added to an answer, you should flag it for moderation attention and ask for it. However I think posting a friendly & polite comment would be much more productive than the notice, why not just do that instead?
Several sites have faced issues with unreferenced answers, and some have opted to add a "back it up" rule in their FAQ, for example check out The Workplace's FAQ:
How should I answer?
Make sure your answer adds helpful information and is a complete, stand-alone answer. Read other answers first and be sure not to completely restate information that has already been posted.
Please note that answers should be backed up either with a reference, or experiences that happened to you personally. You should always include in your answer information about why you think your answer is correct.
If you are having troubles on Islam.SE with lots of opinion based answers, I'd suggest you start a Meta discussion about adding a similar section in the site's FAQ, or a more general discussion on how questions on the site should be answered.