Truly, O Satyakama, this very OM is both the higher and the lower Brahman. Therefore the wise one, through this support alone, attains the one or the other.
Prashna Upanishad
प्रश्नोपनिषद्
मूल श्लोकः
एतद्वै सत्यकाम परं चापरं च ब्रह्म यदोङ्कारः। तस्माद्विद्वानेतेनैवायतनेनैकतरमन्वेति॥
etad vai satyakāma paraṁ cāparaṁ ca brahma yad oṅkāraḥ tasmād vidvān etenaivāyatanenaikataram anveti
PrU 5.2