Jaiminīya (Talavakāra) Brāhmaṇa 1.203

'Come to our pressed [soma] with your bay horses'—these are two-footed Gāyatrī verses. Indra wished to slay Vṛtra. He ran to Prajāpati, [saying,] 'Let me slay Vṛtra!' To him he gave this Anuṣṭubh verse containing the word 'apahara'. With it he did not lay him low. In that after having failed to lay him low he cried out, it became the Nānada. That is the nature of the Nānada. Therefore they say: 'The Nānada should not be made the Ṣoḍaśin sāman, for he did not lay him low with it.' He ran to him again, [saying,] 'Let me indeed slay Vṛtra!' Having fashioned the fiery energy of the seven priestly offices, he gave it to him: three for the Hotṛ's office. It became twenty-one-fold, the Ṣoḍaśin. With it he slew Vṛtra. He who knows thus slays his hateful rival. The twenty-one-fold indeed is the domain of the Ṣoḍaśin. Seven hotrās recite the vaṣat call at the morning pressing, seven at the midday pressing, seven at the third pressing. Moreover, it is that fiery energy that he fashioned and gave to him.

Gaurīviti Śāktya saw this surplus of the sacred word.

It became the Gaurīvita.

What is the Gaurīvita is the surplus of the sacred word; the Ṣoḍaśin is the surplus among the chants.

In that they chant with the Ṣoḍaśin by means of the Gaurīvita, they place surplus upon surplus for the consistency of the sacrifice.