Manava Dharmashastra 1.72

1.72

devayugasāhasraṃ brāhmamekamahaḥ | tāvatī brahmaṇo rātrirdevayugasahasrameva | parisaṅkhyayā saṅkhyānena yatsahasramiti sambandhaḥ | ślokapūraṇārthaścāyamanuvādaḥ | na hyasaṅkhyayā sahasrādivyavahāraḥ | hetau tṛtīyā |

The ‘Time-cycles’ of the Gods, one thousand in number, should be regarded as one ‘day’ of brahmā; and (his) ‘night’ also is of the same extent.—(72)

Medhātithi’s commentary (manubhāṣya):
Thousand ‘Time-cycles’ of the Gods make one ‘day’ of Brahmā.—Brahmā’s night also ‘is of the same extent’—i.e., as long as thousand ‘Time-cycles’ of the Gods.—‘In number’ —i.e., which in computation, is one thousand; the adding of this term is only for the purpose of tilling up the verse; for a thing cannot be spoken of as a‘thousand’ except in number.

The instrumental ending (in ‘Sāṅkhyayā’) denotes causality.