Vedavyasasmriti 86

86

rajodarśanato yāḥ syūrātrayaḥ ṣor̤aśarttavaḥ .
tataḥ puṃvījamakliṣṭaṃ śuddhe kṣetre prarohati .. 2.42 ..

The sixteen nights from the appearance of the menses are the fertile season; / then the male seed grows without affliction in a pure field.