What is the ruling on the prayer led by an Imam who recites the word "الضالين" in Surat Al-Fatihah by separating the silent Lam from the Kasra-ed Lam ("wa-la-ḍ-ḍāl-līn"), knowing that the Lam in it is a mushaddadah (doubled letter) by agreement of all Qira'at (recitations)?

Reciting the word "al-ḍāllīn" (الضالين) with the dissimilation of the idgham (merger of similar letters) does not invalidate the prayer (ṣalāh), because it is a melodic error (laḥn) that does not change the meaning, as mentioned by the jurists. The prayer performed behind someone who makes a melodic error is valid, though it is recommended to advise them.