What Krishna says to Arjuna in the verse 2.19 of the Bhagavad Gita, when both of them are at the battlefield of Kurukshetra, is very close to what Yama, the God of Death, says in the verse 2.19 of the Katha Upaniṣad.
Here’s the verse 2.19 of the Bhagavad Gita:
य एनं वेत्ति हन्तारं यश्चैनं मन्यते हतम् |
उभौ तौ न विजानीतो नायं हन्ति न हन्यते ||
(Neither of them is in knowledge—the one who thinks the soul can slay and the one who thinks the soul can be slain. For truly, the soul neither kills nor can it be killed.)
Here’s Valerie Roebuck’s translation of verse 2.19 from the Katha Upaniṣad:
‘If the slayer thinks it slays;
If the one who is slain thinks it is slain:
Neither of them understands.
It does not slay, nor is it slain.
Taking inspiration from these verses in the Bhagavad Gita and Katha Upaniṣad, Ralph Waldo Emerson has written a poem called "Brahma":
If the red slayer think he slays,
Or if the slain think he is slain,
They know not well the subtle ways
I keep, and pass, and turn again.
Far or forgot to me is near;
Shadow and sunlight are the same;
The vanished gods to me appear;
And one to me are shame and fame.
They reckon ill who leave me out;
When me they fly, I am the wings;
I am the doubter and the doubt,
I am the hymn the Brahmin sings.
The strong gods pine for my abode,
And pine in vain the sacred Seven;
But thou, meek lover of the good!
Find me, and turn thy back on heaven.