How to redirect my website to another website only on mobile devices?

I have a website www.unicodea.com and I am gonna create its subdomain app.unicodea.com. So I want that whenever user opens www.unicodea.com in a mobile device it should redirect to app.unicodea.com. I am repeating this should happen only in mobile devices. PS: This website is made with WordPress. If any plugin or some code is available for this then please provide it.