What is the ruling on praying behind an Imam who does not pronounce the word "Al-Sirat" correctly, but rather pronounces it "Al-Sarat"?

What the questioner mentioned, if it is merely a doubt, should be disregarded, and prayer behind the Imam is valid. However, if the questioner is certain that the Imam drops the shaddah from the word "الصراط" (As-Sirat), then this is a clear grammatical error (Lahn Jali) that invalidates the recitation and the prayer, unless the Imam is unable to learn. As for the congregants praying behind him, if they are proficient in reciting Al-Fatiha, some say their prayer is invalidated, while others say it is valid with dislikedness (Karahah).