Could it be that the Prophet, sallallaahu 'alayhi wa sallam, moved his finger in the prayer due to old age and not being able to keep it still, like it was reported from 'Aa'ishah, that he, sallallaahu 'alayhi wa sallam, would sit in the prayer in his old age? This would explain why Waa'il, a Companion from Yemen who spent little time with the Prophet, sallallaahu 'alayhi wa sallam, in the last years of his life reported it while those who spent their entire lives praying alongside him, some of whom were scholars (including Ibn 'Umar, who was very keen in following the Prophetic Sunnah) did not mention it. Furthermore, there are many reports that mention pointing to indicate Allah's Oneness, and none of them mention movement. Could this be the reason some of the Companions confirmed, "And he would not move it"?
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.
As far as we know, none of the scholars said what you mentioned, but they differed in considering the narration as authentic. Those who considered the narration to be authentic accepted it, and those who considered it weak or odd did not accept it. Please, refer to fataawa 90388 and 124820.
In addition to this, the statement itself is obviously invalid because the Prophet did not reach this state of this imagined weakness to a point that he was unable to keep his finger stable while pointing it. How can this be while the Prophet used to lead the people in prayer standing until his illness in which he died and he only sat in supererogatory prayers in order to prolong the prayer, and he used to perform jihaad, Hajj and make da’wah (proselytization) and run the affairs of Madeenah without any weakness in doing all of this; so how can it be imagined that he was unable to keep his finger stable? Therefore, this statement is obviously false.
Hence, we urge everyone to stop where the scholars have stopped and not innovate statements arising from inexperience about the Sunnah, even if they think that such statements are correct, because it is very serious to make such claims about the religion of Allaah, and making statements about religion without knowledge is one of the grave major sins.
Allaah knows best.
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