Śatapatha Brāhmaṇa ("of a hundred paths") 6.6.4.4

6.6.4.4

atha yadāsmai vratam prayacanti | atha vrate nyajya samidhamādadhāti na vrate nyañjyādityu haika āhurāhutiṃ tajjuhuyādanavakLptaṃ vai tadyaddīkṣita āhutiṃ juhuyāditi

And when they give him (the Sacrificer) the fast-milk, he puts on a kindling-stick, after dipping it into the fast-milk. Some, however, say, 'Let him not dip it into the fast-milk: he would be offering a libation, and it would be improper were one who is initiated to offer a libation.'