What is the ruling on the prayer of one who makes a mistake in reciting a letter of Surat Al-Fatiha, such as not pronouncing the hamza in the word (Ihdina)?

The prayer of one who drops a letter from Al-Fatihah is invalid, unless he is unable to pronounce it. In this case, his prayer is valid for himself, but it is not permissible to follow him in prayer except for someone like him (who also drops letters due to inability).

We should note that the hamza of "ihdina" (اهدنا) is a hamzat wasl (connecting hamza), which is dropped when connecting, and pronounced when stopping at the word "nasta'in" (نستعين).