Śatapatha Brāhmaṇa ("of a hundred paths") 3.8.3.26

3.8.3.26

tadyaddhiraṇyaśakalāvabhito bhavataḥ | ghnanti vā etatpaśuṃ yadagnau juhvatyamṛtamāyurhiraṇyaṃ tadamṛta āyuṣi pratitiṣṭhati tathāta udeti tathā saṃjīvati tasmāddhiraṇyaśakalāvabhito bhavataḥ

The reason why there is a piece of gold on each side is this. When they offer up the victim in the fire, they slay it, and gold means immortal life: thereby then it rests in immortal life; and so it rises from hence, and so it lives. This is why there is a piece of gold on each side.