Among Vedantins; Advait is a widely followed philosophy. The famous quote of Shree Adi Shankaracharya goes:
Brahma Satya, Jagat Mithya.
Now many Advaitins have mentioned that the verse was not concocted by Adi Shankara but he just quoted it from Niralambopanishat.
is said to be taken from ShukaRahasyopanishat.
However many sub-schools of Vedanta blame Adi Shankara and criticize His philosophy of being Crypto-Buddhist and so on. One thing I've often seen is them questioning the authenticity of these two Upanishads viz. Niralambopanishat and ShukaRahasyopanishat.
However as long as I know Niralambopanishat is also one of the 108 main upanishads (Ashtottar-Shat Upanishads)
So I'm interested in knowing if there are any references made to these Upanishads prior to Adi Shankaracharya.