Praise be to Allah.
What is mentioned in the question about there being a story of the origin of the tashahhud and that it happened during the ascent of our Prophet (peace and blessings of Allah be upon him) to heaven (the Mi‘raaj) is something for which there is no basis in Islam.
The scholars of the Standing Committee were asked: Is the tashahhud that we recite in the prayer what the Messenger of Allah (peace and blessings of Allah be upon him) said when he prostrated at Sidrat al-Muntaha during the Mi‘raj?
They replied: It was narrated that Ibn Mas‘ood (may Allah be pleased with him) said: “The Messenger of Allah (peace and blessings of Allah be upon him) taught me the Tashahhud, holding my hand between his hands, just as he would teach me a Surah from the Quran: ‘At-tahiyyatu Lillahi wa’s-salawatu wa’t-tayyibat, as-salamu ‘alayka ayyuha’n-Nabiyyu wa rahmat-Allahi wa barakatuhu. As-salamu ‘alayna wa ‘ala ‘ibad-Illah is-saliheen. Ash-hadu an la ilaha ill-Allah wa ash-hadu anna Muhammadan ‘abduhu wa rasooluhu (All compliments, prayers and pure words are due to Allah. Peace be upon you, O Prophet, and the mercy of Allah and His blessings. Peace be upon us, and on the righteous slaves of Allah. I bear witness that none has the right to be worshipped except Allah, and I bear witness that Muhammad is His slave and Messenger).’
This was narrated by the jama‘ah. According to another version, the Prophet (peace and blessings of Allah be upon him) said: “When one of you sits during the prayer, let him say: al-tahiyyatu Lillahi…” … and when he came to the words, “wa ‘ala ‘ibad-Illah al-saaliheen”, he said: “If you do that, you will have greeted with salam every righteous slave of Allah in heaven and on earth.” And at the end he said: “Then let him choose whatever he wants to ask for.” Agreed upon.
Ahmad narrated from Abu ‘Ubaydah, from his father ‘Abd-Allah ibn Mas‘ood (may Allah be pleased with him) that he said: The Messenger of Allah (peace and blessings of Allah be upon him) taught him the tashahhud and instructed him to teach it to the people: “Al-tahiyyatu Lillahi…” and he quoted it.
The hadeeth (narration) of Ibn Mas‘ood (may Allah be pleased with him) is the soundest hadeeth about the tashahhud, and is to be acted upon according to most of the scholars among the Sahabah (Companions) and Tabi‘een (Successors). Abu Bakr al-Bazzar said: It is the soundest hadeeth about the tashahhud. And he said: It was narrated via twenty-odd hadeeths. Among those who were certain about that was al-Baghawi in Sharh al-Sunnah. End quote.
Thus you will know that this format is the soundest that has been proven from the Messenger of Allah (peace and blessings of Allah be upon him).
With regard to whether the Prophet (peace and blessings of Allah be upon him) recited the tashahhud when he was prostrating at Sidrat al-Muntaha on the night of the Mi‘raj, we do not know of any basis for saying that he prostrated in that place on the night of the Mi‘raj. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baz, Shaykh ‘Abd al-Razzaq ‘Afeefi, Shaykh ‘Abd-Allah ibn Ghadyan, Shaykh ‘Abd-Allah ibn Qa‘ood.
Fatawa al-Lajnah al-Daimah, 7/6,7
Thus it becomes clear that it is not permissible to propagate these letters and essays that are not proven, or which contain innovations (bid‘ah) in beliefs or actions; rather no one should propagate them until after checking whether they are proven and sound.
It was narrated that Hafs ibn ‘Asim said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “It is sufficient lying for a man to speak of everything that he hears.” Narrated by Muslim, 5.
And Allah knows best.