User.Identity.Name - which assembly reference brings that into your project?

I can't find out which reference assembly the User.Identity.Name comes from. How do I find that out? I need to add it to a class library rather than the default MVC 5 app I have made so that I can have access to it?

http://msdn.microsoft.com/en-us/library/system.web.httpcontext.user(v=vs.110).aspx

C# Class Library / AUser.cs

public class AUser
{
    public string Username
    {
        get
        {
            return User.Identity.Name // doesn't notice User
        }
    }
}