Jaiminīya (Talavakāra) Brāhmaṇa 1.202

Regarding this they say: 'The Ṣoḍaśin should not be performed at an Ukthya. The ukthas are cattle; the Ṣoḍaśin is the thunderbolt. He would turn the thunderbolt against cattle. It is only to be performed at an Atirātra.' Waters are the pacification of everything; with waters they indeed pacify it. If it should be a Soma rite for sorcery, having made a golden thunderbolt having edges, to whomever would perform a rite, toward him he should roll it. That one hurls the thunderbolt at him for his overthrow. When the Soma rite is completed, having overturned the trough, or having made figures of barley groats, or of green grasses, pronouncing their names, he should cut their necks, [saying,] 'Here I cut the neck of so-and-so; here of so-and-so; here of so-and-so.' As many as the names he pronounces, so many necks he cuts; he gives it to the Brahman. The Brahman is indeed the sacred power; the sacred power is the thunderbolt. In the sacred power itself he thereby establishes the thunderbolt.

Indra and the Bṛhat came into being together.

The Bṛhat surpassed Indra by one form.

He feared that [form, thinking,] 'By this very one it will overcome me.'

He said to the gods: 'Let this sacrificial rite be the Ṣoḍaśin.'

'So be it,' [they replied].

It became the Ṣoḍaśin.

That is the origin of the Ṣoḍaśin.

He who knows thus surpasses his hateful rival in prosperity, having chanted on two-footed Gāyatrī verses with the Bṛhat,

[and] having chanted with the Ṣoḍaśin.