The story is not authentic.

i have heard that reciting Surah Al Mulk everynight , will protect from the punishment of the grave. Is this authentic ? Is the hadith below authentic? “One of the companions of the Prophet (ﷺ) pitched a tent on a grave without knowing that it was a grave. Suddenly he heard a person from the grave reciting Surah al-Mulk till he completed it. So he went to the Prophet (ﷺ) and said: ‘Oh Messenger of Allah, I pitched my tent on a grave without realizing that is was a grave. Then suddenly I heard a person from the grave reciting Surah al-Mulk till he completed it.’ The Messenger of Allah (ﷺ) said: ‘It is the defender, it is the deliverer- it delivers from the punishment of the grave.'”