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).