Agniveshyagrihyasutra 2.5.6.18

2.5.6.18

atha viṣṇuṃ dvādaśyāṃ bhagavantamarcayitvā sudarśanenāṣṭasahasraṃ japtvā triḥ paryāyam āvaśyakaḥ putro bhavatītyāha putro bhavatītyāha \

Now, having worshiped the Blessed Lord Viṣṇu on the twelfth lunar day, and having muttered [the mantra of] Sudarśana eight thousand times, [performing this] three times in succession, a son necessarily comes to be; so he said; a son comes to be; so he said.