Shatapatha Brahmana 1.7.1.10

1.7.1.10

atha yavāgvaitāṃ rātrimagnihotraṃ juhoti | ādiṣṭaṃ vā etaddevatāyai havirbhavati yatpayaḥ sa yatpayasā juhuyādyathānyasyai devatāyai havirgṛhītaṃ tadanyasyai juhuyādevaṃ tattasmādyavāgvaitāṃ rātrimagnihotraṃ juhoti juhvatyagnihotramupakLptokhā bhavatyathāhopasṛṣṭām prabrūtādeti yadā prāhopasṛṣṭeti

That night he performs the Agnihotra with rice-gruel (yavâgû). That milk, namely, (which he milks that night) has already been announced as sacrificial food to a (special) deity; hence, if he were to make the offering with milk, he would offer to one deity that which has been set apart as sacrificial food for another deity: this is the reason why on that night he performs the Agnihotra with rice-gruel. As soon as they have performed the Agnihotra, the pot is made ready. He (the Adhvaryu) thereupon says, 'Announce that she (the cow) has been let loose to (the calf)!' When he (or she, the milker) announces, 'She has been let loose!'--