What is the ruling on saying "Masha'Allah" (as God willed) during prayer upon being impressed by the Imam's voice or remembering something?

A Muslim is not permitted to utter human speech during their prayer, because the speech that invalidates prayer is human speech. As for the remembrance of Allah (dhikr) and supplication (du'a), these are not human speech and therefore do not invalidate the prayer. Dhikr in prayer is divided into two types: The first is that which relates to the prayer, such as supplicating when passing by verses of hope or fear. This is recommendable. The second is that which does not relate to the prayer, such as praising Allah when sneezing or seeking refuge (with Allah) upon hearing the braying of a donkey, and so on. This is not recommendable in prayer; rather, it is disliked according to the Hanbalis, but it does not invalidate the prayer. Some jurists have explicitly stated its dislike to avoid the disagreement of those who prohibited it or deemed it to invalidate the prayer. What indicates the general permissibility of this is what came in the hadith of Abu Bakr when he raised his hands and praised Allah during the prayer.

In summary, the congregant's saying "Masha'Allah" (as Allah wills) in prayer, out of admiration for the imam's voice, does not invalidate the prayer, but it should be avoided.