Does the prayer of one who intentionally says "In the name of Allah, what Allah wills" while reciting the Tashahhud silently become invalid, and is he required to repeat it?

Reciting dhikr (remembrance of Allah) during prayer does not invalidate it. However, reciting it during the tashahhud (testimony of faith) is not prescribed, because, according to some jurists, continuity between its phrases is obligatory, and thus it is not permissible to interpose anything else between them. The Shafi'is hold that continuity between the words of the tashahhud is obligatory, and that nothing else, whether dhikr or Qur'an, should be interposed between them. They are of the view that whoever intentionally recites dhikr during the tashahhud has breached this continuity and added something unprescribed, and therefore must repeat the prayer in which this occurred.