Jaiminīya (Talavakāra) Brāhmaṇa 2-8.374

The cows once held a sacrificial session at the beginning, desiring to obtain proper food.
They arose in the tenth month, thinking: 'We have obtained all proper food.'
These are the horned ones.
Some of them, however, said: 'Let us complete all these twelve months.'
They applied themselves excessively.
In the twelfth month, their horns fell away.
These are the hornless ones.
Therefore, in the twelfth month, the participants in a sacrificial session also shave off their tufts of hair,
for then they become like the cows.
They arose [saying]: 'We have prospered!' having obtained the sacrificial session, having obtained all proper food.
Therefore these [cows] stand through all twelve months, both the two rainy [months] and the two winter [months];
for it was conquered by them through the sacrificial session, for it was obtained by them.
Those who, knowing thus, complete the twelve months, arise having obtained all proper food.
The sacrificial session is indeed one whose sacrificial gift is oneself.
By the first two [upasads] they redeem only their hair; by the second two, their skin; by the third two, their flesh; by the fourth two, their bone; by the fifth two, their marrow.
Whatever of them remains unredeemed there, that they redeem by the sixth two.
Having become clean, purified, fit for sacrifice, and holy, they enter the world of the gods.

Prajāpati, indeed, was this [universe] in the beginning.

He desired: 'May I be manifold, may I reproduce, may I attain greatness.'

He practiced austerity.

He saw this Vaiśvānara introductory Atirātra.

From it he brought forth day and night.

He was established upon day and night.

Therefore, that this Vaiśvānara introductory Atirātra takes place,

from this they bring forth day and night, they are established upon day and night.