I think you mean this dialogue cited by Shankaracharya in the Brahmasutra Bhashya 3.2.17 -
बाष्कलिना च बाध्वः पृष्टः सन् अवचनेनैव ब्रह्म प्रोवाचेति श्रूयते — ‘स होवाचाधीहि भो इति स तूष्णीं बभूव तं ह द्वितीये तृतीये वा वचन उवाच ब्रूमः खलु त्वं तु न विजानासि । उपशान्तोऽयमात्मा’ इति ।
The meaning is: Bashkali approached Badhva for Brahmavidya: He asked 'Please instruct me'. The latter remained silent. The aspirant persisted, for the second and the third time. The Acharya replied: I am replying indeed but you do not get it: The Atman is Quiescence.
The source of the passage is not known.
regards
subrahmanian.v