Why not declare the array as int type (or whatever numeric type the field is defined as)?
int [] pras = new int[] {1, 3, 5};
var authEntity = new AuthenticationEntities();
var prac = (from p in authEntity.Practices
where pras.Contains(p.PracticeID)
select new Practices
{
Practice = p.PracticeName
})
.ToList<Practices>();