What is the ruling on praying behind an imam who recites al-Fatihah incorrectly, substituting sin and sad with shin, and what should the congregation do?

Summary

According to the definitive position of the Shafi'i school, an imam who changes sin or sad to shin is considered ummi because the error alters the meaning. A congregant who recites properly has an invalid prayer behind such an imam and must repeat the prayers performed, to the best of his ability, estimating the number if counting is difficult. If the number is too great to manage and the person did not know the ruling earlier, he may follow the weaker view and other schools that treat it as a minor error, so past prayers need not be repeated. The matter should be reported to the Religious Department, and congregants should not pray behind him until he corrects his recitation or is replaced.

Written in our own words from the original fatwa — check the source before acting on it.