What is the Ruling on Omitting An Ayah from Al-Fatihah in the Prayer?

If a muslim prays and omits (i.e. doesn't recite) an Ayah from Al-Fatihah, his prayer is invalid. (That's) because he didn’t recite Al-Fatihah, regardless of the omitted Ayah being from the beginning, middle, or end of the Surah. Except if it was due to forgetfulness and not intentional. In this case the ruling (of the following ayah) is applied to him, “Our Rabb! Punish us not if we forget or fall into error.” [Al-Baqarah, 286]