Katha Upanishad - Verse 2.1.11
Text: मनसैवेदमाप्तव्यन्नेह नानास्ति किंचन । मृत्योः स मृत्युं गच्छति य इह नानेव पश्यति ॥ ११ ॥ manasaivedamāptavyanneha nānāsti kiṃcana | mṛtyoḥ sa mṛtyuṃ gacchati ya iha nāneva paśyati || 11 || 11. By mind alone could this be obtained, there is no difference here at all. He goes from death to death who sees as if there were difference here. Shankara’s Commentary: Before attaining the knowledge of the oneness, by the mind purified by the sastras and by the preceptor, this unalloyed essence of the Brahman should be attained in the form ‘There is the Brahman alone, nothing else exists....