The Tashahhud - Section 3 Mishkat al-Masabih 917
Arabic hadith text
وَعَنْ نَافِعٍ قَالَ: كَانَ عَبْدُ اللَّهِ بْنُ عُمَرَ إِذَا جَلَسَ فِي الصَّلَاةِ وَضَعَ يَدَيْهِ عَلَى رُكْبَتَيْهِ وَأَشَارَ بِأُصْبُعِهِ وَأَتْبَعَهَا بَصَرَهُ ثُمَّ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «لَهِيَ أَشَدُّ عَلَى الشَّيْطَانِ مِنَ الْحَدِيدِ» . يَعْنِي السبابَة. رَوَاهُ أَحْمد
Translation
Nafi‘ said that when ‘Abdallah b. ‘Umar sat during the prayer he placed his hands on his knees, pointed with his finger and gave his whole attention to it. He then said that God’s Messenger had said, “It has a more severe effect on the devil than iron,” meaning the forefinger. Ahmad transmitted it.