In this excerpt from his book "Spiritual Heritage of India", Swami Prabhavananda quotes a number of passages from the Upanishads concerning the indescribability of Brahman. In particular, he says this:
And again, in a passage preserved only in Shankara's commentary:
'"Sir," said a pupil to his master, "teach me the nature of Brahman." The master did not reply. When a second and a third time he was importuned, he answered: "I teach you indeed, but you do not follow. His name is silence."'
First of all, I'd preferably like to know what Upanishad this passage is taken from, even if the actual text of this Upanishad is no longer available. But failing that, my question is, where in the works of Adi Shankaracharya is this passage quoted?
It says that it's preserved in "Shankara's commentary". Does that mean one of his commentaries on the principal Upanishads, or his commentary on the Brahma Sutras, or what? I can't seem to find the quote outside of Swami Prabhavananda's book (or sources that cite it).