In this answer, a strange and rather fascinating story is quoted from the Seder Hadoros:
Seder Hadoros (year 4930) brings the story of an argument between Rabeynu Tam and other Rishonim regarding how one should tie the knot of the Tefillin. Eventually, Moshe Rabeynu himself is called down from heaven and asked his opinion. He testifies that he personally saw the back of Hashem's Tefillin shel Rosh, but Rabeynu Tam is not persuaded. In the words of the Seder Hadoros: "Rabeynu Tam got up like a lion and said Moshe Rabeynu you are mistaken!".
Does anyone have a source for this story, and/or any record of discussion regarding the veracity and validity of this story?