Maybe it is only a story...if you ask where it is found, in which purana, which chapter, which slokam.. No answer.
We know AdiShakarabhagavatPadaa wrote a commentary on Vishnusahasranamam.. Judging by the clear attachment of Shankara for the smartha way of life, this is an enigma.
There is a story abot it..
One day the Acharya was planning to compose a commentary on Lalitha Sahasranamam.. He asked his disciple Sureshwarachary to fetch the palm-leaves containing the text of Lalithasahasranamam from the inside of the Mutt..
The disciple went inside and brought a bundle of palm-leaves and to his surprise it was Vishnusahstranamam..
The Acharya pointed out the mistake and sent the disciple to fetch the correct text.
This time when the disciple went in he saw a cute girl of eight years red in colour and clad in red and this girl handed over a bundle of palm-leaves saying this that must be the book the Guru was searching for..
Dutifully Sureshwaracharya took the bundle, and when Acharyaswamikal opened it it was found that Vishnusahasranamam alone has been brought once again ..
The Guru asked the disciple as to why the wrong text was brought once again.
And Sureshwara replied that a small girl child was standing inside the mutt and had given him this text and insisted that it should be taken to the Guru ..
The acharya could not understand what was happening.
." A young girl in our mutt?.".
So the Master went in to verify what was happening, with Sureshwara following closely.. He saw the Girl of the hue of Sindooraaruna, clad in dark red dress, and she was telling.
" You describe the names of my brother, I am pleased to hear that more than mine.."
It was Gauri Lalithamahatripurasundari, instructing Shankara.. He prostrated at Her lotus feet and started explaining Vishnusahasranamam..
The great Bhagavatha exponent and Tapaswi , Brahmasri Sengalipuram Anantharama Deeskhitar has narrated this story in many of his discourses..