What did the Messenger of Allah, peace and blessings be upon him, say in the Tashahhud: "I bear witness that there is no god but Allah, and I bear witness that I am the Messenger of Allah," or "I bear witness that Muhammad is the Messenger of Allah"?

The Prophet, peace and blessings be upon him, used to testify with "I bear witness that Muhammad is the Messenger of Allah" or "I bear witness that Muhammad is His servant and His Messenger." It was not surprising for him to mention his noble name; rather, it is the fundamental practice. This has been narrated in the Sunnah, as in his sermon reported by Muslim. He also recited the Quran as it was revealed, with "Muhammad is the Messenger of Allah," without alteration, which confirms that he performed the prescribed invocations for his nation in their original forms.