As others have mentioned, bounties are Stack Exchange's tool for this purpose. Placing a bounty costs some reputation, but it will have the following effect:
- The question is immediately bumped to the top of the active question list.
- The question gets placed in the home page's featured tab for seven days.
both of which attract more visitors who (potentially) will upvote the answer (and as a side-effect, perhaps your question as well?). A week later, you can 'award' the bounty to that answer (you can even do it earlier if you like, but then it won't be featured anymore) and the author of the answer will gain the reputation you've invested in the bounty.
While most bounties are placed in search for a new/better answer, one of the predefined reasons for a bounty is
Reward existing answer
One or more of the answers is exemplary and worthy of an additional bounty.
so this is absolutely fine.
As an alternative, you can link to the answer in e.g. your profile page, on social media or on other Mathematics sites on the Internet (local rules may apply).