عَنْ نَافِعٍ قَالَ: كَانَ عَبْدُ اللَّهِ بْنُ عُمَرَ رَضِيَ اللَّهُ عَنْهُ إِذَا جَلَسَ فِي الصَّلَاةِ وَضَعَ يَدَيْهِ عَلَى رُكْبَتَيْهِ وَأَشَارَ بِإِصْبَعِهِ وَأَتْبَعَهَا بَصَرَهُ، ثُمَّ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ: ((لَهَا أَشَدُّ عَلَى الشَّيْطَانِ مِنَ الْحَدِيدِ)) يَعْنِي الْسَّبَّابَةَ
Nafi' says: When Sayyiduna Abdullah bin Umar (may Allah be pleased with him) would sit in prayer, he would place his hands on his knees, point with his finger, and keep his gaze fixed upon it. Then he would narrate that the Messenger of Allah (peace and blessings be upon him) said: Verily, this finger is harder upon Satan than iron.