How to use group by with union in T-SQL

How can I using group by with union in T-SQL? I want to group by the first column of a result of union, I wrote the following SQL but it doesn't work. I just don't know how to reference the specified column (in this case is 1) of the union result.

SELECT  *
FROM    ( SELECT    a.id ,
                    a.time
          FROM      dbo.a
          UNION
          SELECT    b.id ,
                    b.time
          FROM      dbo.b
        )
GROUP BY 1