Is the prayer of one who erred in articulating the Tashahhud by saying "La ilaha illa Allah" instead of "Ashhadu an la ilaha illa Allah" valid, or must it be repeated? And does this articulation change the meaning of the words?

A Muslim must correct his prayer as reported from the Prophet, may Allah bless him and grant him peace. An unintentional error in reciting the Qur’an, or the adhkar (remembrances), or the tashahhud (testification of faith) in prayer, if it does not change the meaning, does not invalidate the prayer. The worshipper should rectify it by correcting it. The error mentioned here (omitting "an" from "Ashhadu an la ilaha illa Allah") does not change the meaning, and therefore does not harm the prayer. One should not preoccupy oneself with the whispers of correcting past prayers, but rather strive to learn the Sunnah and correct prayer and worship in the future.