Śatapatha Brāhmaṇa ("of a hundred paths") 5.3.5.28

5.3.5.28

atha bāhū vimārṣṭi | mitrasyāsi varuṇasyāsīti bāhvorvai dhanurbāhubhyāṃ vai rājanyo maitrāvaruṇastasmādāha mitrasyāsi varuṇasyāsīti tadasmai prayacati tvayāyaṃ vṛtram badhediti tvayāyaṃ dviṣantam bhrātṛvyam badhedityevaitadāha

He then strokes the two arms, with, 'Mitra's thou art,--Varuna's thou art;' for the bow is within the two arms, and by his two arms the Râganya pertains to Mitra and Varuna: therefore he says, 'Mitra's thou art, Varuna's thou art.' He hands it to him, with, 'May he slay Vritra by thee!' whereby he means to say, 'May he slay by thee his spiteful enemy!'