What is the reason of raising the index finger of the right hand while sitting after performing the two prostration/sajood of the second and final unit/rakat of the prayer?
What is the source of this practice? And is it mandatory?
The wisdom behind this is that by rising the index finger in salat, it is harmful to Shaytaan in the sense that he is being humiliated and what he invites to. Ibn Ummar mentioned when he raised his finger in salat he said that the Prophet (peace and blessings be upon him said):
لهي أشد على الشيطان من الحديد يعني السبابة
it is definitely more affective/harmful on Shaytaan then metal
No, it is not mandatory to to raise it but Mustahab.