Khandogya-Upanishad: Second Prapathaka, Thirteenth Khanda
[The next Khanda is not translated by Muller: this translation from The Principal Upanishads, S. Radhakrishnan tr.]
1. One summons, that is the syllable him. He makes request, that is a prastava. Along with the woman, he lies down, that is the udgiha. He lies on the woman, that is the pratihara. He comes to the end, that is the nidhana. He comes to the finish, that is the nidhana. This is the Vamadevya chant woven on sex intercourse.
2. He who knows this Vamadeva chant as woven on sex intercourse, comes to intercourse, procreates himself from every act, reaches a full length of life, lives well, becomes great in offspring and in cattle, great in fame. One should not despise any woman. That is the rule.