KAṬHA UPANISHAD · 2.1.11

मनसैवेदमाप्तव्यं नेह नानाऽस्ति किंचन ।
मृत्योः स मृत्युं गच्छति य इह नानेव पश्यति ॥ ११॥

manasaivedamāptavyaṃ neha nānā’sti kiṃcana ।
mṛtyoḥ sa mṛtyuṃ gacchati ya iha nāneva paśyati ॥ 11॥

Through the mind must we understand that there is nothing in this world that is really various; who thinketh he sees difference here, from death to death he goeth.