Katha Upanishad - Verse 2.2.1
Text: पुरमेकादशद्वारमजस्यावक्रचेतसः । अनुष्ठाय न शोचति विमुक्तश्च विमुच्यते । एतद्वैतत् ॥ १ ॥ puramekādaśadvāramajasyāvakracetasaḥ | anuṣṭhāya na śocati vimuktaśca vimucyate | etadvaitat || 1 || 1. The city of the unborn, whose knowledge is permanent, has eleven gates; thinking on him, one does not grieve and being freed, becomes free. This verily is that. Shankara’s Commentary: As Brahman is not easily knowable, this if commenced for the purpose of ascertaining the entity of the Brahman, again by another method....