Shatapatha Brahmana 5.3.5.15

5.3.5.15

atha pavitre karoti | pavitre stho vaiṣṇavyāviti so 'sāveva bandhustayorhiraṇyam pravayati tābhyāmetā abhiṣecanīyā apa utpunāti tadyaddhiraṇyam pravayatyamṛtamāyurhiraṇyaṃ tadā svamṛtamāyurdadhāti tasmāddhiraṇyam pravayati

He then prepares two strainers (pavitra), with (Vâg. S. X, 6), 'Purifiers ye are, Vishnu's own;'--the significance is the same (as before). He weaves gold (threads) into them. With them he purifies those consecration waters. As to why he weaves gold (threads) in,--gold is immortal life: that immortal life he lays into these (waters), and hence he weaves gold (threads) in.