Jaiminīya (Talavakāra) Brāhmaṇa 5.16

5.16

atha saṃhitaṃ dvyakṣaraṇidhanaṃ pratiṣṭhāyai
dvipādvai yajamānaḥ pratiṣṭhityai
ayā pavasva devayuḥ | pavate haryato harir
ityuṣṇikkakubhau bhavataḥ
gāyatryau rūpeṇa rāthantare sabhaṃ pūrvasyāṃ vijityā eva
athākṣaraṃ padanidhanamapariṣṭobhaṃ rāthantaram
tasmādrāthantare'hankriyate
prajāpatirvā etaṃ yajñamasṛjata yaddvādaśāham
tasya tṛcairanyānyahānyakalpantaikarcairetat
tadasya samivāplīyata
sa ete sāmanī apaśyat
tābhyāmenadyathā śāṃkubhyāṃ carma vihanyādevamevābhyāmetadaharvyahan
āśvena purastādākṣāreṇopariṣṭāt
tadyadete sāmanī bhavato'hna eva dhṛtyai
pra sunvānāyāndhasa
iti pravatīranuṣṭubho bhavanti rāthantare'han
praṇinīṣeṇyaṃ vā etadahaḥ
preti gāyatryai rūpam
gāyatryā evaitadrūpeṇa prayanti

Then [there is] the Saṃhita [sāman] with a two-syllable finale for firm support. Two-footed is the sacrificer, for firm support. "Flow here, seeking the gods; the golden-hued horse flows"—these are an Uṣṇih and a Kakubh. They are two Gāyatrīs by form on a Rathantara [day], [...] for conquering the first [day]. Then [there is] the Ākṣāra [sāman], having a word-finale, without stobhas, belonging to the Rathantara. Therefore it is performed on a Rathantara day. Prajāpati created this sacrifice, namely the twelve-day rite. Its other days were formed with triplets, but this one with single verses. Thus it collapsed, as it were. He saw these two sāmans. With them, just as one would peg out a hide with two pegs, even so with these two he pegged out this day: with the Āśva in front, with the Ākṣāra behind. In that these two sāmans are present, it is for the holding fast of the day. "Forward for him who presses the food"—thus there are Anuṣṭubh verses containing [the word] "forward" (pra) on a Rathantara day. For this day is one on which they wish to advance. "Forward" is the form of the Gāyatrī. With the form of the Gāyatrī they advance.