Bhagavad Gita 8.17

Those who understand the cosmic day and night know that one day of creation is a thousand cycles, and that the night is of equal length.

सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदुः।
रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः॥

sahasrayugaparyantamaharyadbrahmaṇo viduḥ|
rātriṃ yugasahasrāntāṃ te'horātravido janāḥ||

English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).