Is the Adhan (call to prayer) divinely ordained and unalterable (tawqifi), and is the letter "nun" in "Ashhadu an la ilaha illa Allah" pronounced with a shaddah (doubled) or sukoon (unvowelled)? And what is the ruling on one who deviates from the commonly accepted Adhan, which pronounces the "nun" with a sukoon, and instead pronounces it with a shaddah, likening it to the "nun" in "Ashhadu anna Muhammadan Rasool Allah"?

The Adhan is a prescribed act of worship, so no additions to it are permissible without evidence. The word "an" (أَن) in "Ashhadu an la ilaha illa Allah" (I bear witness that there is no god but Allah) is obligatorily lightened, because its subject is implicit and its predicate is a sentence. It is not correct to liken it to "anna" (أَنَّ) in "Ashhadu anna Muhammadan Rasoolullah" (I bear witness that Muhammad is the Messenger of Allah), for that is an invalid analogy due to the difference in the state of the subject and predicate in both phrases.