Is it permissible for the one praying behind the Imam to recite when the Imam prolongs his recitation?

It is not permissible for the congregant in a loud prayer to recite anything more than Al-Fatihah. Rather, he must listen attentively to the Imam's recitation after Al-Fatihah. An exception to this is the recitation of Al-Fatihah due to the Hadith: "There is no prayer for one who does not recite the Fatihah of the Book," and due to the generality of Allah Almighty's saying: "And when the Qur'an is recited, then listen to it and pay attention that you may receive mercy," and due to the Prophet's (peace and blessings be upon him) saying: "When the Imam recites, then listen attentively."