Does the prayer become invalid if the worshiper forgets the definite article "al-" when reciting Al-Fatiha, such as by saying "hamdullah" instead of "Alhamdulillah"?

The Fajr prayer mentioned is not invalidated according to the majority of scholars, so it does not need to be repeated. This is because they hold that the recitation of Al-Fatiha is waived for the congregant (ma'mum) in this situation.

However, the Shafi'is believe that reciting Al-Fatiha is a pillar (rukn) for both the congregant and others. Therefore, whoever omits even one letter of it and does not rectify it, their prayer is invalid and must be repeated.

The most cautious approach is to repeat the prayer, out of consideration for the opinion of those who deem it invalid.