Matsya Purana 60.50

60.50

idamiha madanena pūrvamiṣṭaṃ śatadhanuṣā kṛtavīryasūnunā ca.
kṛtamatha varuṇena nandinā vā kimu jananātha tato yadudbhavaḥ syāt.. 60.49 ..

Cupid was the first to observe this ordinance, and Sahasrabâhu, the son of Kârtavîrya, was the second to observe it ; and afterwards it was kept up by Varuṇa and Nandi, and consequently this is considered so sacred in this world.—49.