The Rahasyatraya Sara is a work by the Sri Vaishnava Acharya Vedanta Desikan, concerning the meaning of the Rahasya Traya or three secrets of the Sri Vaishnava sect (of which I'm a member). (One of the secrets is the Dvaya mantra used to surrender to Vishnu, which I discuss here.) In this excerpt from that work, Vedanta Desikan discusses the importance of a guru choosing his disciples wisely:
From the episodes of Raikwa (and others) it is evident that the guru who does not reveal spiritual truths immediately even to those shishyas who are of excellent character will not be adversely affected in his spiritual life. If, on the other hand, the guru reveals these spiritual truths to any and every disciple in the belief that the shishya that sought him must be good, he will be adversely affect; for, in such cases, it is said that the sins of the disciple become the sins of the guru.
This may be seen from the story of Brahma. Without careful inquiry (into his fitness) he revealed truths to Indra. As a consequence Brahma forgot his divine knowledge and had to be taught again by Bhagavan through the agency of Narada, Brahma's very disciple.
My question is, what is the story of Brahma losing his divine knowledge after teaching it to Indra, and then his son Narada restoring it?
Narada is an incarnation of Vishnu, so this is similar to Vishnu's incarnation Hayagriva teaching Brahma the Vedas after they'd been stolen from him. In any case, the only reference I've found to this story is in this article:
Brahma failed to test Indra, when the latter came to Brahma for instruction. Because Brahma taught Brahma vidya to an undeserving student, Brahma forgot the Brahma vidya!
But does anyone know what scripture this story comes from? There's a story in the Chandogya Upanishad about Brahma instructing Indra and the Asura Virochana (son of Prahlada), but in the end of that story Indra turned out to be a more worthy disciple than Virochana. But is it possible that he still wasn't sufficiently worthy to be taught by Brahma?