Bhagavad Gita · 8.17

सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदुः।रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः॥

sahasra-yuga-paryantam ahar yad brahmaṇo viduḥ rātriṁ yuga-sahasrāntāṁ te 'ho-rātra-vido janāḥ

Those who know that Brahma's day lasts a thousand ages and his night also lasts a thousand ages — they truly understand day and night.

— Bhagavad Gita 8.17

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)