Jaiminīya (Talavakāra) Brāhmaṇa 57.297

Indra, having hurled the thunderbolt at Vṛtra, thinking, 'I have not struck [him] down,' went to the farthest distance. The farthest distance, indeed, is the anuṣṭubh; into it he entered. He pushed out twelve of its syllables eastward and twelve upward. Then into that eight-syllable gāyatrī which was in the middle, into that he entered; into that very one he entered. That was its twelve syllables. The gāyatrī is brahman; it was brahman itself that he entered into the middle. He pushed this year around on both sides. These verses are indeed Indra's cup. By resorting to Indra's cup, they conclude the rite. Therefore, one who is performing sorcery or one who has a rival should perform these. These verses are indeed Indra's cup; having resorted to Indra's cup, one overcomes his hateful rival.

In safety, indeed, he is consecrated, in safety he sacrifices, in safety he concludes the rite—he who, knowing thus, praises with these verses. They make this anuṣṭubh cryptically. If they were to make it visibly an anuṣṭubh, the sacrificers would go to the farthest distance and would not find firm support. That they make it cryptically an anuṣṭubh is for firm support.