Is it obligatory for the congregant to correct the imam's mistake in recitation during prayer, and is it valid to correct it after prayer?

It is permissible for the congregant to prompt his Imam if the latter stops or makes a mistake in recitation, as has been narrated from the Prophet (peace be upon him) and his companions. If the error is in Al-Fatiha, by omitting one or more verses, then the prayer is not valid unless the omitted part is recited, due to the Prophet's (peace be upon him) saying: "There is no prayer for one who does not recite the Opening of the Book." However, if the omission is from other than Al-Fatiha, or if the error is a grammatical mistake (lahn) outside of Al-Fatiha, then the prayer is valid. If the grammatical mistake in Al-Fatiha alters the meaning and is not corrected, then the prayer becomes invalid. It is fitting that the one who leads the prayer should be a skilled reciter.