Isn't it the sunnah of prophet to lower your index finger and raise it?
The clearly established sunna is to point with the index finger during the tashahhud. As for raising the index finger or moving it, there are conflicting narrations about it which has led to the different schools having different stances about it. One narration says the Prophet 
did not move his index finger while another mentions that he moved it. Because of this the different schools have different opinions on whether to do it or not. But keeping the finger still or moving it are both acceptable according to the Mālikī scholar Ibn ʿAbd al-Barr.
The Ḥanafī opinion is to point during the tashahhud and lift the finger when you say lā ilāha illā allāh then lower it again.
The Mālikī opinion is to continuously move the finger from the beginning to the end of the tashahhud.
The Ḥanbalī opinion is to lift the finger each time the name of God is mentioned in the tashahhud.
The Shafiʿi opinion is to lift the finger when you say illā allāh and to keep it lifted until the end of the tashahhud.
There is no conclusive evidence for any of these options.
Sources:
- Qatari fatwa (Arabic PDF)
- Qatari fatwa 2 (Arabic PDF)