As salaam aleykum In Awjaz ul Masalik it is stating and he is quoting it from a book called Kashf al Ghammah: Sayyiduna Abdullah bin Zubair used to say that the Prophet (SAW) would not move his forefinger except at the time of pointing, and by pointing he sought to be signify sincerity and the oneness of Allah. However he doest give a source for it,having asked this to my brother he said: that Kashf al Ghammah might be a Shia book, and if that is the case most probably the hadeeth is fabricated. You know more about it? And where is this hadeeth recorded with these words? Was salaam

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The book "Awjaz Al-masaalik Ilaa Muwatta' Maalik" is authored by the prominent scholar Muhammad Zakariyyaa Al-Kandahlawi . When discussing the issue of moving the index in Tashahhud, he quoted the words which were mentioned in the question from the book "Kashful Ghummah."

However, saying that "Kashful Ghummah" may be from the books of Shiites is very unlikely because the author is a well-known Sunni scholar, and it is impossible that he would report something from the Raafidhah (Shiites) books.

Furthermore, the Raafidhah are more unlikely to report or give evidence for their actions or statements based on the actions and statements of the companions, like Ibn Az-Zubayr .

Nonetheless, we think that the book which is mentioned in the question is the book entitled "Kashful Ghummah 'An Jamee' Al-ummah" authored by the prominent scholar 'Abdul-Wahhaab Ash-Sha'raani . This is a book on jurisprudence from which the scholars usually quote.

As regards the evidence of the narration of Ibn Az-Zubayr, it is reported by Abu Daawood and An-Nasaa'ee as narrated by 'Abdullaah Ibn Az-Zubayr that the Prophet used to point with his index finger when supplicating but he never used to move it.

However, the narration of Waa'il Ibn Hujr is contradictory to the above as he described the method of the prayer of the Prophet and mentioned the position of putting the hands in Tashahhud, saying: "…Then he raised his (index) finger, and I saw him moving it while supplicating." [Abu Khuzaymah and Al-Bayhaqi] 

Allaah Knows best.