All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
When one joins the congregational prayer late while the Imaam (the one who leads others in prayer) is standing during a Rak‘ah (unit of prayer); saying the opening supplication in such a case is controversial and the scholars provided some details. If he knows that saying the supplication would not make him miss reciting Al-Faatihah, he can say it; but if he knows or predominantly thinks that saying this supplication would make him miss reciting Al-Faatihah, he is required to recite Al-Faatihah instead. Such details are applied to the inaudibly-recited prayer. As for the audibly-recited prayer, one is required to listen to the Imaam when he starts recitation, and he is required to say nothing but reciting the Soorah of Al-Faatihah.
Finally, we point out that the opening supplication is recommended. It is not obligatory at all.