It seems to me, and Allah knows best, that if you have already prayed one rak`ah with its two prostrations, then you should complete your prayer quickly, because of the hadith of Abu Hurayrah (may Allah be pleased with him), that the Prophet (peace and blessings of Allah be upon him) said: “Whoever catches up with one rak`ah of the prayer has caught up with the prayer.” (Agreed upon).
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, because of the obligation to listen to the khatib .
And Allah knows best.
