Shatapatha Brahmana 4.3.2.3

4.3.2.3

prajāpatirvā udgātāḥ | yoṣargghotā sa etatprajāpatirudgātā yoṣāyāmṛci hotari retaḥ siñcati yatstute taddhotā śastreṇa prajanayati tacyati yathāyam puruṣaḥ śitastadyadenacyati tasmācastraṃ nāma

Now the Udgâtri is Pragâpati, and the Hotri, (being) the Rik (fem.), is a female. And when he chants, then the Udgâtri, Pragâpati, implants seed in the female Hotri, the Rik; this the Hotri brings forth by means of the sastra (recitation), he sharpens it even as this man is sharpened, and because he thereby sharpens (so) therefore it is called sastra.