Nagarjuna's footprints

[Yigal Bronner]

Dear Indology members,
There is a story narrating how Nagarjuna takes on an invisible form to
enter and view the women at some king's harem, only to be detected by the
footprints his invisible feet leave on the flowers scattered (for that
purpose) on the harem's floor.
Can anyone refer me to a textual location of this story?
Yigal Bronner.

Yigal Bronner
Ph.D. candidate
Department for South Asian languages and Civilizations
University of Chicago.