What is the correct wording for the Tashahhud, and is it permissible to say: "I bear witness that there is no god but Allah, and I bear witness that Muhammad is the Messenger of Allah," or is it obligatory to suffice with saying: "I bear witness that there is no god but Allah and Muhammad is the Messenger of Allah"? And what is the story behind the Tashahhud?

What you have heard about the impermissibility of saying, "And I bear witness that Muhammad is His slave and His Messenger," is false. The Prophet, peace and blessings be upon him, taught his Companions the Tashahhud (definition) with this wording, as is authentically reported in the hadiths of Ibn Mas'ud and Ibn Abbas, agreed upon by Al-Bukhari and Muslim. The disagreement among scholars regarding the wordings of the Tashahhud is not about permissibility but about what is more virtuous and preferred. As for the story of the Tashahhud on the Night of Mi'raj, it is fabricated and has no basis in the books of Sunnah (definition).