Shatapatha Brahmana 5.5.1.11

5.5.1.11

atha yaiṣā maitrāvaruṇī payasyā bhavati | tasyai vaśā dakṣiṇā sā hi maitrāvaruṇī yadvaśā yadi vaśāṃ na vindedapi yaiva kā cāpravītā syātsarvā hyeva vaśāpravītā tāmadhvaryubhyāṃ dadāti prāṇodānau vā adhvaryū prāṇodānau mitrāvaruṇau tasmāttāmadhvaryubhyāṃ dadāti

And what dish of curds there is for Mitra-Varuna, the fee for that is a sterile cow, for that one is sacred to Mitra-Varuna. If he cannot procure a sterile cow, any unimpregnated one will do; for every sterile cow is indeed unimpregnated. He gives it to the two Adhvaryus; for the Adhvaryus are the out-breathing and the in-breathing, and the out-breathing and in-breathing are Mitra-Varuna: therefore he gives it to the two Adhvaryus.