If you have already prayed one rak`ah with its two prostrations, then you should complete your prayer quickly. But if the khatib starts his khutbah before you have completed the first rak`ah with its two prostrations, then you should stop your prayer.

Praying Zuhr after Jumu`ah is a bid`ah and a going astray, instituting a new way which Allah has not prescribed. We must not do this; we must beware of it and warn people against it