By reciting this zikr, you are safe from the punishment or torment from Angels Zabaniah who takes care of the gates of Hell (19 in total). If you notice, the khalimah contains 19 letters (huruf) that hold the keys to protect from these 19 Angels.
One day, the ruler of Rome, King Qaisar, a kafir, wrote a letter to Amirrul Mukminin, Khalifah Saidina Umar Ibn Khattab, complaining about how he often suffer from a severe headache, and never get to recover after all attempts at trying all the medicine he could possibly think of.
Upon hearing this, Saidina Umar took a fez cap and deliver it to the King to be worn by him. With Allah swt will, as soon as the King wore that fez cap, he recovers from his illness. What is strange after that, is when, as soon as he took off the cap, he experienced the headache back. "This is weird", said the King. Then, he ordered for the fez cap to be checked and found a piece of paper folded inside the cap.
When he unfold it, he saw the khalimah, Bismilillahirahmanirrahim. He was moved by the wisdom of the khalimah that heals him from his illness, and amazed by Allah swt grace, that he, put both of his palm up (in gesture of making du'a) and say the Shahadah.Subhanallah!