What is the ruling on uttering "الضالين" (the misguided) by merely opening the mouth at the alif without producing a sound, and is the prayer valid with that?

The prayer is not valid by merely pronouncing the letter 'Ḍād' in "al-ḍāllīn" in Surat al-Fatihah without following it with the long vowel 'Alif' (madd), because whoever omits a letter from al-Fatihah, their prayer is not valid, due to the Prophet's (peace be upon him) saying: "There is no prayer for one who does not recite the Opening of the Book." This is only unless one is unable to pronounce it, in which case their prayer is valid for themselves.