Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.4.8

3.2.4.8

atha yaddhruvāyāmājyam pariśiṣṭam bhavati | tajjuhvāṃ catuṣkṛtvo vigṛhṇāti barhiṣā hiraṇyam prabadhyāvadhāya juhoti kṛtsnena payasā juhavānīti samānajanma vai payaścahiraṇyaṃ cobhayaṃ hyagniretasam

In the first place he pours the butter, which remains in the dhruvâ spoon, in four parts into the guhû; and having tied a piece of gold with a blade of the altar-grass, and laid it down (in the guhû), he offers (the butter), thinking, 'I will offer with pure milk;' for milk and gold are of the same origin, since both have sprung from Agni's seed.