Shatapatha Brahmana 6.3.1.42

6.3.1.42

tāṃ haike hiraṇmayīṃ kurvanti | hiraṇyayīti vā abhyukteti na tathā kuryādyadvā eṣā candāṃsi tenaiṣā hiraṇyamamṛtaṃ hiraṇyamamṛtāni candāṃsi

Some, indeed, make it of gold, saying, 'It is spoken of as golden.' Let him not do so: in that it is the metres, thereby that (spade) is gold, immortal gold, the immortal metres.