Need some clarification

[Sreekanth Sathayanarayana]

Namaste

I was having a discussion with some friends of mine (about advaita) and one of
them told that Sri shankaracharya in one of his texts, had written the
sentence--" eshah vaidika dharmah". There was a debate on to where it was
written (I have not done any extensive study of Sri Sankara's works). Finally no
one could point to the source. Do any of the learned members on the list know
about the source of this saying from Sankaracharya's works? Please let me know,
in which chapter and text did Shankarachaarya write this particular sentence?

Thanks
Sreekanth