All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Imaam Ahmad reported that when ‘Abdullaah ibn ‘Umar sat in prayer [i.e. in the position of Tashahhud] he would put his hands on his knees and he would point with his (index) finger and he would look at it [finger] and then he said: "the Prophet said: “It (meaning the index finger) is more powerful against the devil than iron.” The meaning –according to what the scholars said –is that pointing to the Oneness of Allaah is more powerful against the devil than iron because he is not harmed by iron like he is harmed by the Oneness of Allaah, therefore, this pointing is harder for the devil than the sword and the arrow because of the Oneness [of Allaah] that it refers to. This, indeed, makes the devil give up hope in the praying person to commit Shirk [associate partners to Allaah] and disbelief. Mujaahid said about this pointing: ‘It is a hooked rod (of iron) i.e. a humiliation and torture for the devil.'
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Allaah Knows best.