Jaiminīya (Talavakāra) Brāhmaṇa 1.193

1.193

ekākṣaraṃ devānāavamamāsītsaptākṣaraṃ paramam
navākṣaramasurāṇāmavamamāsītpañcadaśākṣaraṃ paramam
te devā akāmayanta kanīyasā bhūyo'surāṇāṃ vṛñjīmahīti
ta ekākṣareṇa pañcadaśākṣaramavṛñjata
te dvyakṣareṇa caturdaśākṣaraṃ tryakṣareṇa trayodaśākṣaraṃ caturakṣareṇa dvādaśākṣaraṃ
pañcākṣareṇaikādaśākṣaraṃ ṣaḍakṣareṇa daśākṣaraṃ saptākṣareṇa navākṣaramaṣṭākṣareṇāṣṭau
tāni yatṣoḍaśaṣoḍaśa samapadyanta tatṣoḍaśinaṣṣoḍaśitvam
tadgāyatryeva saptadaśyupasamapadyata
tasmādāhussaptadaśaḥ prajāpatiriti
indro vai vṛtramajighāṃsat
sa prajāpatimupādhāvaddhanāni vṛtramiti
tasmā etaṃ ṣoḍaśinaṃ vajraṃ prāyacchadyadasya vīryamāsīttadādāya śakvaryaḥ
taṃ nāśaknoddhantum
taṃ punarupādhāvaddhanānyeva vṛtramiti

The lowest of the gods was one-syllabled, the highest seven-syllabled. The lowest of the Asuras was nine-syllabled, the highest fifteen-syllabled. The gods desired: 'May we with the lesser wrest away the greater belonging to the Asuras.' With the one-syllabled they wrested away the fifteen-syllabled; with the two-syllabled the fourteen-syllabled; with the three-syllabled the thirteen-syllabled; with the four-syllabled the twelve-syllabled; with the five-syllabled the eleven-syllabled; with the six-syllabled the ten-syllabled; with the seven-syllabled the nine-syllabled; with the eight-syllabled, eight. Because these amounted to sixteen each, that is the sixteen-ness of the Ṣoḍaśin. It amounted to seventeen through the Gāyatrī. Therefore they say: 'Prajāpati is seventeenfold.' Indra indeed wished to slay Vṛtra. He ran up to Prajāpati, saying: 'May I slay Vṛtra.' To him he gave this Ṣoḍaśin as a thunderbolt, having taken what vital power he had, namely the Śakvarī verses. He was not able to slay him. He ran up to him again, saying: 'May I slay Vṛtra.'