What is the ruling on the prayer of the Imam and the congregants, and are they required to repeat it, if the Imam inadvertently omitted one of the verses of Al-Fatihah and did not realize it until after the prayer concluded and a large number of worshipers had dispersed?

If the Imam realizes he omitted something from Al-Fatiha before a long separation, it suffices for him and those with him to perform one rak'ah instead of the deficient rak'ah, along with the prostrations of forgetfulness (sujud as-sahw), and their prayer will be valid. However, if he does not realize until after a long separation, then only his prayer is invalidated, not the prayer of the congregants (ma'mumeen). "Long" here refers to exiting the mosque or remaining in it for a period customarily considered long. According to the Malikis, if no "long separation" occurs, then the prostrations of forgetfulness are prescribed before the tasleem, along with repeating the prayer as a precaution.