Śatapatha Brāhmaṇa ("of a hundred paths") 3.8.2.27

3.8.2.27

tadyaddhiraṇyaśakalāvabhito bhavataḥ | ghnanti vā etatpaśuṃ yadagnau juhvatyamṛtamāyurhiraṇyaṃ tadamṛta āyuṣi pratitiṣṭhati tathāta udeti tathā saṃjīvati tasmāddhiraṇyaśakalāvabhito bhavata āśrāvyāhāgnīṣomābhyāṃ cāgasya vapām medaḥ preṣyeti na prasthitamityāha prasute prasthitamiti vaṣaṭkṛte juhoti

The reason why there is a piece of gold on both sides is this. When they offer the victim in the fire they slay it; and gold means immortal life: hence it (the victim) rests in immortal life. And so it rises from hence, and so it lives; for this reason there is a piece of gold on both sides. Having called for the Sraushat, he says (to the Maitrâvaruna), 'Prompt (the Hotri to recite the offering-prayer on) the omentum and fat of the buck for Agni and Soma!' He does not say, '... (the omentum and fat) brought forward;' when the Soma has been pressed he says, 'brought forward.' He offers when the Vashat has been pronounced.