Shatapatha Brahmana 6.7.1.14

6.7.1.14

prādeśamātryūrdhvā bhavati | prādeśamātro vai garbho viṣṇuryonireṣā garbhasammitāṃ tadyoniṃ karotyaratnimātrī tiraścī bāhurvā aratnirbāhuno vai vīryaṃ kriyate vīryasammitaiva tadbhavati vīryaṃ vā etaṃ yantumarhati vīryeṇaitaṃ devā abibharurvīryeṇaivainametadbibharti

It (the seat) is a span high; for Vishnu, as an embryo, was a span high: he thus makes the womb equal in size to the embryo. It is a cubit across; for the cubit is (the length of) the (fore-)arm, and strength is exerted by the arm. It thus is made equal to strength, and strength is indeed capable of sustaining him (Agni): by means of strength the gods did bear him, and by means of strength does he now bear him.