Bhagavad Gita 8.17 Chapter 8, Verse 17

सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदुः ।
रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः ॥८.१७॥

sahasrayugaparyantamaharyadbrahmaṇo viduḥ
rātriṃ yugasahasrāntāṃ te'horātravido janāḥ

Meaning

Those who know that a day of Brahma lasts a thousand ages and his night another thousand ages truly understand day and night.

Newly written by Bhajan Dhun — scholarly review in progress.

Reference: Bhagavad Gita 8.17 (Chapter 8, The Yoga of the Imperishable Absolute, Verse 17)