BṚHADĀRAṆYAKA UPANISHAD · 6.4.3

तस्या वेदिर् उपस्थः ।
लोमानि बर्हिश् ।
चर्माधिषवणे ।
समिद्धो मध्यतस् तौ मुष्कौ ।
स यावान् ह वै वाजपेयेन यजमनस्य लोको भवति तावान् अस्य लोको भवति ।
य एवं विद्वान् अधोपहासं चरत्य् आसां स्त्रीणां सुकृतं वृङ्क्ते ।
अथ य इदम् अविद्वान् अधोपहासं चरत्य् आस्य स्त्रियः सुकृतं वृञ्जते ॥

tasyā vedir upasthaḥ ।
lomāni barhiś ।
carmādhiṣavaṇe ।
samiddho madhyatas tau muṣkau ।
sa yāvān ha vai vājapeyena yajamanasya loko bhavati tāvān asya loko bhavati ।
ya evaṃ vidvān adhopahāsaṃ caraty āsāṃ strīṇāṃ sukṛtaṃ vṛṅkte ।
atha ya idam avidvān adhopahāsaṃ caraty āsya striyaḥ sukṛtaṃ vṛñjate ॥

Her lap is a sacrificial altar; her hairs, the sacrificial grass; her skin, the soma-press. The two labia of the vulva are the fire in the middle. Verily, indeed, as great as is the world of him who sacrifices with the Vâjapeya ('Strength-libation') sacrifice, so great is the world of him who practises sexual intercourse, knowing this; he turns the good deeds of women to himself. But he who practises sexual intercourse without knowing this-women turn his good deeds unto themselves.

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