Śatapatha Brāhmaṇa ("of a hundred paths") 5.4.3.27

5.4.3.27

agreṇa maitrāvaruṇasya dhiṣṇyam | maitrāvaruṇī payasyā nihitā bhavati tāmasya bāhū abhyupāvaharatīndrasya vāṃ vīryakṛto bāhū abhyupāvaharāmīti paśūnāṃ vā eṣa raso yatpayasyā tatpaśūnāmevāsyaitadrasam bāhū abhyupāvaharati tadyanmaitrāvaruṇī bhavati mitrāvaruṇā u hi bāhū tasmānmaitrāvaruṇī bhavati

In front of the Maitrâvaruna's hearth the dish of curds for Mitra and Varuna has been deposited. He draws down to it his (the Sacrificer's) two arms, with, 'I draw you down, the arms of Indra, the doer of mighty deeds.' Now curds are the essence of cattle: hence it is to the essence of cattle that he thereby brings down his (the Sacrificer's) arms. And as to its being for Mitra-Varuna, it is because Mitra and Varuna are the two arms.