Srimad Bhagavatam Canto 10, Chapter 87, Text 03

SB 10.87.3

saisa hy upanisad brahmi
 purvesam purva-jair dhrta
srraddhaya dharayed yas tam
 ksemam gacched akiñcanah
Those who came before even our ancient predecessors meditated upon this same confidential knowledge of the Absolute Truth. Indeed, anyone who faithfully concentrates on this knowledge will become free from material attachments and attain the final goal of life.
This confidential knowledge concerning the Absolute Truth should not be doubted, since it has been passed down through authoritative lines of learned sages from time immemorial. One who cultivates the science of the Supreme with reverence, avoiding the distractions of fruitive rituals and mental speculation, will learn to give up the false designations of material body and mundane society, and thus he will become eligible for perfection.
In the opinion of Srila Visvanatha Cakravarti, the first two verses of this chapter can be considered an Upanisad on the topic of Brahman. Sukadeva Gosvami here disclaims authorship on the grounds that this Upanisad was spoken previously by Narada Muni, who himself heard it from Sanaka Kumara.
Srimad Bhagavatam Canto 10, Chapter 87, Text 02
Srimad Bhagavatam Canto 10, Chapter 87, Text 04