Who are the people whose heads are crushed with rocks, then return to their original state, whom the Prophet (peace and blessings be upon him) saw?

The summary of the answer:

He is a man whom Allah taught the Qur’an, but he slept through it at night and did not act upon it during the day. This is his torment in the Barzakh until the Day of Judgment, as stated in the Hadith of the Prophet, peace and blessings be upon him, narrated by Al-Bukhari from Samurah ibn Jundub.