Mundaka Chapter 2 Section 2 Verse 1 to 2

 2.2.1 द्वौ पक्शौ वृक्षे, एकः खादति मीठं अम्लं
Dvau pakṣau vṛkṣe, ekaḥ khādati mīṭhaṁ amlaṁ
Two birds sit on the same tree: one eats sweet and bitter fruits.

2.2.2 अन्यः केवलं दर्शयति, निर्लिप्तः
Anyaḥ kevalaṁ darśayati, nirliptaḥ
The other merely observes, detached (the Supreme Self).