What is the ruling on a worshipper led by an Imam (ma'mum) omitting the recitation of Surat Al-Fatihah in a silent prayer, or in the last two rak'ahs of an audible prayer? And is there a difference between intentional omission and forgetfulness?

The preponderant view is the obligation for the congregant (ma'mum) to recite Surat Al-Fatihah in every rak'ah of the prayer, whether it is a silent or audible prayer. This is based on the generality of the Prophet Muhammad’s (peace be upon him) saying: "There is no prayer for one who does not recite the Opening of the Book (Al-Fatihah)," and his saying: "Do not do so except with the Opening of the Book, for there is no prayer for one who does not recite it." These two hadiths specify the hadith: "And when he recites, then listen attentively."

This is the new view of the Shafi'i school, the view of Al-Bukhari and the Zahiriyyah, and the choice of many verifying scholars. If the congregant omits Al-Fatihah, their prayer is invalidated according to the preponderant view. If it is omitted by forgetfulness, they must repeat the rak'ah or the prayer. As for the latecomer (masbuq) who joins the imam during the bowing (ruku'), it is not prescribed for him to recite Al-Fatihah.