Is it true that the Prophet, peace and blessings be upon him, used to move his index finger in a circular motion throughout the entire Tashahhud until the Taslim, and that he would raise his finger when saying, "I bear witness that there is no god but Allah, and that Muhammad is His slave and His Messenger," and then resume the circular motion? And would the questioner be sinful if he adopted this opinion?

The default is to trust the words of the father if he is a student of knowledge interested in studying Islamic sciences. Regarding the Tashahhud (definition), Muslim narrated that the Messenger of Allah, peace and blessings be upon him, when he sat for it, would place his left hand on his left knee and his right hand on his right knee, forming a circle with three fingers and fifty, and pointing with his index finger.