As mentioned in Brihadaranyaka Upanishad 4.4.19
मनसैवानुद्रष्टव्यं नेह नानास्ति किञ्चन । मृत्योः स मृत्युमाप्नोति य इह नानेव पश्यति ॥
Through the mind alone (Atman) is to be realised. There is no difference whatsoever in It. He goes from death to death ( cycle of birth and death), who sees difference, as it were, in It.
मनसा means mind purified by the knowledge of the supreme Truth, and in accordance with the instructions of the teacher
Same Shruti is also found in Kathopanishad 2.1.10 -11
यदेवेह तदमुत्र यदमुत्र तदन्विह।
मृत्योः स मृत्युमाप्नोति य इह नानेव पश्यति ॥
What is in this world is also in the other, and what is in the other, that again is in this; who think he sees difference here, from death to death he goes.
मनसैवेदमाप्तव्यं नेह नानाऽस्ति किंचन।
मृत्योः स मृत्युं गच्छति य इह नानेव पश्यति ॥
Through the mind must we understand that there is nothing in this world that is really various; who think he sees difference here, from death to death he goes.