Śatapatha Brāhmaṇa ("of a hundred paths") 2.3.1.3

2.3.1.3

atha yadastameti | tadagnāveva yonau garbho bhūtvā praviśati taṃ garbham bhavantamimāḥ sarvāḥ prajā anu garbhā bhavantīlitā hi śere saṃjānānā atha yadrātristira evaitatkaroti tira iva hi garbhāḥ

And when he sets, then he, as an embryo, enters that womb, the fire; and along with him thus becoming an embryo, all these creatures become embryos; for, being coaxed, they lie down contented. The reason, then, why the night envelops that (sun), is that embryos also are, as it were, enveloped.