Moving the forefinger during the Tashahhud
Question

What is the detailed explanation for moving the forefinger during the Tashahhud, and is there a hadeeth about that?
 
May Allah reward you with good and benefit the ummah by your knowledge.
Praise be to Allah.It is Sunnah to point with the forefinger and move it during the Tashahhud, because of the report narrated by Ahmad (18890) and al-Nasaa’i from Waa’il ibn Hajar who said: I said, I will watch how the Messenger of Allah (peace and blessings of Allah be upon him) prays. So I watched him and he stood up and said “Allahu akbar,” and raised his hands until they were level with his ears, then he placed his right hand over his left and left wrist and left forearm. Then when he wanted to bow he raised his hands likewise. He put his hands on his knees, then when he raised his head he raised his hands likewise. Then he prostrated and put his hands level with his ears, then he sat resting on his left foot and placed his left hand on his thigh and left knee, and he put the elbow of his right hand on his right thigh. Then he made a circle with two of his fingers, then he raised his finger and I saw him moving it and making du’aa’ with it.” This hadeeth was classed as saheeh by al-Albaani in Saheeh al-Nasaa’i. 
Muslim (580) narrated that ‘Abd-Allah ibn ‘Umar said: When he – meaning the Prophet (peace and blessings of Allah be upon him) – sat during the prayer, he would place his right hand on his right thigh and clench all his fingers, and point with the finger that is next to the thumb, and he would place his left hand on his left thigh. 
According to a report narrated by Muslim from Ibn ‘Umar, when the Prophet (peace and blessings of Allah be upon him) sat during the prayer, he would put his hands on his thighs and raise the finger of his right hand that is next to the thumb, and make du’aa’ with it, and his left hand would be resting on his left thigh. 
Al-Nasaa’i (1273) narrated that Sa’d said: The Messenger of Allah (peace and blessings of Allah be upon him) passed by me whilst I was making du’aa’ with my fingers and he said, “One, one,” and pointed with his index finger. Meaning: point with one finger, namely the index finger. 
Ahmad (5964) narrated that Naafi’ said: When ‘Abd-Allah ibn ‘Umar sat during the prayer, he would put his hands on his thighs and point with his finger whilst looking at it. Then he said: The Messenger of Allah (peace and blessings of Allah be upon him) said, “It is harder on the Shaytaan than iron,” meaning the index finger. This hadeeth was classed as hasan by al-Albaani in Sifat Salaat al-Nabi (peace and blessings of Allah be upon him), p. 159. 
These ahaadeeth indicate two things: 
1 – That one should point with the finger throughout the entire Tashahhud
2 – That one should move it when making du’aa’. 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) explained the phrases narrated for the du’aa’ in the Tashahhud. He said: 
Every time you make du’aa’ then move your finger towards upwards towards the One upon Whom you are calling, so we say: 
Al-salaamu ‘alayka ayyuha’l-Nabiyyu (Peace be upon you O Prophet) – you should point your finger whilst reciting this because the salaam is a kind of du’aa’. 
Al-salaamu ‘alayna (peace be upon us) – point your finger. 
Allahumma salli ‘ala Muhammad (O Allah, send prayers upon Muhammad) – point your finger. 
Allahumma baarik ‘ala Muhammad (O Allah, send blessings upon Muhammad) – point your finger. 
A’oodhu Billaaih min ‘adhaabi Jahannam (I seek refuge with Allah from the torment of Hell) – point your finger. 
Wa min ‘adhaab il-qabr (and from the torment of the grave) – point your finger. 
Wa min fitnat il-mahyaa wa’l-mamaat (and from the trials of life and death) – point your finger. 
Wa min fitnat il-maseeh il-dajjaal (and from the trial of the Dajjaal) – point your finger. 
Every time you make du’aa’ point upwards towards the One upon Whom you are calling. This is closer to the Sunnah. 
End of quotation from al-Sharh al-Mumti’, 3/202 
And Allah knows best.