Originally Posted by Samiu

The Hanafi scholars say that the ruling regarding raising the index finger is from the Hadith as follows:

It is narrated that when Rasulullah (sallallahu alaihi wasallam) used to sit down to supplicate, (to recite tashahhud) he used to place his right hand on his right thigh and his left hand on his left thigh. He would indicate at the time of reciting the shahadah by raising his index finger. He would also join the ends of his thumb and middle finger (thereby forming a circle). [Sahih Muslim, chapter on the description of sitting-Hadith no.579]