BṚHADĀRAṆYAKA UPANISHAD · 6.4.11

अथ याम् इच्छेद् दधीतेति तस्याम् अर्थं निष्ठाय मुखेन मुखं संधायापान्याभिप्राण्यात् ।
इन्द्रियेण ते रेतसा रेत आदधामीति ।
गर्भिण्य् एव भवति ॥

atha yām icched dadhīteti tasyām arthaṃ niṣṭhāya mukhena mukhaṃ saṃdhāyāpānyābhiprāṇyāt ।
indriyeṇa te retasā reta ādadhāmīti ।
garbhiṇy eva bhavati ॥

Now, the woman whom one may desire with the thought, 'May she conceive!'--after coming together with her and joining mouth with mouth, he should first exhale, then inhale, and say: 'With power, with semen, I deposit semen in you!' Thus she becomes pregnant.

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