What is the ruling on saying: (I bear witness that there is no god but Allah) instead of (I bear witness that there is no god but Allah) in the Tashahhud?

If the question is about reciting (Ashhadu an la ilaha illa Allah) by pronouncing the silent 'n' and not assimilating it into the 'l', this does no harm, and the prayer is valid. It is mentioned in "Nihayat al-Zayn" that pronouncing the assimilated 'n' in "an la ilaha illa Allah" or the assimilated tanwin in "Muhammadan Rasul Allah" does not cause harm according to the relied-upon opinion, because it does not omit a letter but rather pronounces the assimilated one.