Shatapatha Brahmana 14.1.4.16

14.1.4.16

atha patnyai śiro'pavṛtya mahāvīramīkṣamāṇām vācayati tvaṣṭṛmantastvā sapemeti vṛṣā vai pravargyo yoṣā patnī mithunamevaitatprajananaṃ kriyate

Thereupon he uncovers the head of the (Sacrificer's) wife, and makes her say whilst she is looking at the Mahâvîra, 'Together with Tvashtri will we serve thee: (bestow thou sons and cattle upon me! bestow thou offspring upon us! may I remain unscathed together with my husband!)'--the Pravargya (m.) is a male, and the wife is a female: a productive pair is thus produced.