What is the authenticity of Raja' ibn Abi Salamah's statement regarding the manner in which Adam and Iblis descended, and is it permissible to clasp one's hands and raise one's head if the statement is incorrect?

The narration regarding the descent of Adam and Iblis in this manner is from Raja' bin Abi Salama. It is a report concerning the unseen and likely originates from Isra'iliyyat (Judeo-Christian traditions). Another report from Ibn Abi Shaybah mentions that he disliked shortening the prayer because Iblis was cast down in a shortened form. Neither of these reports constitutes a proof, as they are not attributed to the Prophet, peace and blessings be upon him.

As for interlocking fingers, it is disliked during prayer, while walking to prayer, or while waiting for prayer, with some difference of opinion among the Salaf (pious predecessors). However, outside of prayer, it is permissible. The general rule for customs is permissibility unless there is evidence to the contrary. Imam Al-Bukhari dedicated a chapter to the permissibility of interlocking fingers in the mosque and elsewhere, and he narrated hadiths that support this, thus confirming its permissibility outside of prayer.