Gobhilagrihyasutra 3.10.19

3.10.19

tāṃ sandhivelāsamīpaṃ purastād agner avasthāpyo, 'pasthitāyāṃ juhuyād yat paśavaḥ pra dhyāyate 'ti;

Shortly before the time of junction (of day and night, i.e. before the morning twilight) he should place that (cow) to the east of the fire, and when (that time) has come, he should sacrifice (Âgya) with (the verse), 'What, O beasts, you think' (MB. II, 2, 5).