What is the ruling on the prayer of an Imam who recites (ihdina) with an open hamza, and the prayer of the congregants behind him? And is it obligatory to repeat the prayer while informing the congregants?

If a melodic error (laḥn) in Al-Fatiḥah alters the meaning, the prayer becomes invalid. Scholars have differed on whether opening the hamzat al-waṣl (connecting hamza) in "Ihdinā" constitutes a melodic error that alters the meaning, with two opinions:

1. The sound opinion within the Ḥanbalī school, and that adopted by Ibn ʿUthaymīn, is that it is a melodic error that alters the meaning. This is because "Ahdinā" (أهدنا) with an open hamza (fatḥah) comes from "al-ihdāʾ" (إعطاء الهدية), meaning "giving a gift," whereas "Ihdinā" (اهدنا) with a hamzat al-waṣl comes from "al-hidāyah" (الدلالة والتوفيق), meaning "guidance and success." 2. It was also said that opening it does not alter the meaning.

Based on the opinion that it is a melodic error that alters the meaning, the prayer is not valid and must be repeated. Informing the worshippers about this is good, but not obligatory.