The Prophet (peace and blessings of Allaah be upon him) said: "It is more powerful against the Shaytaan than iron (reported by Imaam Ahmad, 2/119, with a hasan isnaad, as stated in Sifat al-Salaah, p. 159), i.e., pointing with the forefinger during the Tashahhud is more painful to the Shaytaan than being beaten with a rod of iron, because it reminds the slave of the Unity of Allaah and to be sincere in his worship of Him alone, and this is what the Shaytaan hates most; we seek refuge with Allaah from him. (al-Fath al-Rabbani by al-Saa'idi, 4/15).

Because of this great benefit, the Sahaabah, may Allaah be pleased with them, used to enjoin one another to do this and were very keen to remember to do this thing which so many people nowadays take so lightly. It was reported that the Companions of the Prophet (peace and blessings of Allaah be upon him) used to enjoin one another, i.e., with regard to pointing with the finger during the du'a. (Reported by Ibn Abi Shaybah with a hasan isnaad, as stated in Sifat al-Salaah, p. 141. See al-Musannaf, no. 9732, part 10, page 381, Dar al-Salafiyyah, India, edn.)

The Sunnah in pointing with the forefinger, is that it should be raised up and down, moving, pointing towards the qiblah, throughout the Tashahhud.