Praise be to Allah.
The Sunnah is for the worshipper to point with his forefinger when reciting the tashahhud, because of the report narrated by Muslim (580) from Ibn ‘Umar, according to which, when the Prophet (blessings and peace of Allah be upon him) sat in prayer, he placed his hands on his knees and raised his right finger that is next to the thumb [i.e., the forefinger], offering supplication with it, and his left hand was spread on his [left] knee.
According to another report, when the Messenger of Allah (blessings and peace of Allah be upon him) sat to recite the tashahhud, he placed his left hand on his left knee, and placed his right hand on his right knee, and made a shape with his fingers to indicate the number fifty-three, and pointed with his index finger.
It says in Sharh Muntaha al-Iraadaat (1/201): He pointed with the index finger of his right hand, raising it without moving it.
The forefinger is the sabbahah (that which glorifies Allah), because a person points with it to signify Tawheed when reciting the tashahhud and when offering supplication in general – in other words, when praying and otherwise; and when mentioning the name of Allah, may He be exalted, because of the marfoo‘ hadith of ‘Abdullah ibn az-Zubayr, who used to point with his forefinger, without moving it, when he offered supplication. Narrated by Abu Dawood and an-Nasaa’i.
It was narrated that Sa‘d ibn Abi Waqqaas said: The Prophet (blessings and peace of Allah be upon him) passed by me when I was offering supplication with two fingers, and said: “Only one, only one,” and gestured with his forefinger. Narrated by an-Nasaa’i. End quote.
As for the worshippers pointing with the forefinger when the imam recites a verse in which there is veneration of Allah or mention of His beautiful names, there is no basis for that, and it is contrary to the Sunnah, which is to place the right hand over the left when standing, as al-Bukhaari (740) narrated that Sahl ibn Sa‘d said: The people were instructed that a man should place his right hand over his left forearm when praying. Abu Haazim said: I believe that he attributed that to the Prophet (blessings and peace of Allah be upon him).
So the worshippers should be informed that there is no basis for this action, and that it is contrary to the Sunnah of placing the right hand over the left arm, and that doing that as an act of worship comes under the heading of innovations that have been introduced into the religion (bid‘ah).
And Allah knows best.