Kauṣītaki Upaniṣad 4.12.1

4.12.1

sa hovāca bālākir ya evaiṣa chāyāyāṃ puruṣas tam evāham upāsa iti /

Bâlâki said: 'The person that is in the echo, on him I meditate.'
Agâtasatru said to him: 'Do not challenge me on this. I meditate on him as the second, who never goes away. Whoso meditates on him thus, he gets a second from his second (his wife), he becomes doubled.
Bâlâki said: 'The sound that follows a man, on that I meditate.
Agâtasatru said to him: 'Do not challenge me on this. I meditate on him as life. Whoso meditates on him thus, neither he himself nor his offspring will faint before the time.'