Is there any فضْل (merit/virtue) mentioned in relation to the dhikr: "Lā ilāha illallāh, wa lā naʿbudu illā iyyāh" (There is no god but Allah, and we worship none but Him)?

What you asked about is part of a Hadith reported in Sahih Muslim from Abu Al-Zubayr, that Ibn Al-Zubayr used to say after every prayer, when he finished it: "There is no god but Allah, alone, without partner. To Him belongs all sovereignty and all praise, and He is over all things competent. There is no might nor power except with Allah. There is no god but Allah, and we worship none but Him. To Him belong all blessings, all grace, and all excellent praise. There is no god but Allah, we are sincere in devoting our religion to Him, even if the disbelievers detest it." And he said: "The Messenger of Allah, peace and blessings be upon him, used to pronounce these (words of) glorification after every prayer." No authentic Hadith has been found specifying a particular virtue for this remembrance, other than it being one of the remembrances authentically transmitted to be recited after the obligatory prayers.