Whenever Abdullah bin Umar sat during the prayer, he would place his hands on his knees, point with his finger, and follow it with his gaze. Then he would say, "The Messenger of Allah ﷺ said, 'It is harder on Satan than iron,'" meaning the index finger.
كان عبد الله بن عمر إذا جلس في الصلاة وضع يديه على ركبتيه، وأشار بإصبعه، وأتبعها بصره ثم قال: قال رسول الله ﷺ: لهي أشد على الشيطان من الحديد يعني السبابة.
Imam Ahmad (600) and Bazzar - Kashf Astar (563) both narrated it through the route of Muhammad bin Abdullah Abu Ahmad Zubayri, who said, Kathir bin Zayd narrated to us, from Nafi, and he mentioned something similar.Kathir bin Zayd is Aslami, then Sahmi, their freedman, Abu Muhammad Madani. There is disagreement about him. Nasai criticized him. As for Ibn Main, his students differed about his view of him. Abdullah bin Dawraqi reported from him, "There is no harm in him." Muawiya bin Salih and others reported from him, "He is acceptable (salih)." Ibn Abu Khuthaymah reported from him, "He is not much." Ibn Ammar Mawsili deemed him trustworthy, and Abu Zura, Ahmad, Ibn Adi and others regarded him favorably.Someone like this, his hadith is graded hasan, especially when supported by corroborating narrations. This is why Bayhaqi (2, 132) did not say anything about him, but rather criticized Muhammad bin Umar Waqidi, who reported from Kathir bin Zayd the wording, "Moving the finger during prayer frightens off Satan." This route was also narrated by Ruyani in his Musnad (1439) and Ibn Adi in Kamil (6/4247).Bayhaqi said, "Muhammad bin Umar Waqidi alone narrated it, and he is not strong." He also said, "We have reported from Mujahid that he said, 'A man moving his finger while sitting in prayer repels Satan.'"