BṚHADĀRAṆYAKA UPANISHAD · 6.4.21

अथास्या ऊरू विहापयति -- विजिहीथां द्यावापृथिवी इति ।
तस्याम् अर्थं निष्ठाय मुखेन मुखं संधाय त्रिर् एनाम् अनुलोमाम् अनुमार्ष्टि --
विष्णुर् योनिं कल्पयतु त्वष्टा रूपाणि पिंशतु ।
आ सिञ्चतु प्रजापतिर् धाता गर्भं दधातु ते ।
गर्भं धेहि सिनीवालि गर्भं धेहि पृथुष्टुके ।
गर्भं ते अश्विनौ देवाव् आधत्तां पुष्करस्रजौ ॥

athāsyā ūrū vihāpayati -- vijihīthāṃ dyāvāpṛthivī iti ।
tasyām arthaṃ niṣṭhāya mukhena mukhaṃ saṃdhāya trir enām anulomām anumārṣṭi --
viṣṇur yoniṃ kalpayatu tvaṣṭā rūpāṇi piṃśatu ।
ā siñcatu prajāpatir dhātā garbhaṃ dadhātu te ।
garbhaṃ dhehi sinīvāli garbhaṃ dhehi pṛthuṣṭuke ।
garbhaṃ te aśvinau devāv ādhattāṃ puṣkarasrajau ॥

Then he spreads apart her thighs, saying: 'Spread yourselves apart, heaven and earth!' Inserting the member in her and joining mouth with mouth, he strokes her three times as the hair lies, saying:-- 'Let Vishṅu make the womb prepared! Let Tyashtri shape the various forms! Prajâpati--let him pour in! Let Dhâtri place the germ for thee! O Sinîvâlî, give the germ; O give the germ, thou broad-tressed dame! Let the Twin Gods implace thy germ-- The Asvins, crowned with lotus-wreaths!

Robert Ernest Hume · The Thirteen Principal Upanishads (1921), Bṛhadāraṇyaka VI.4