Shatapatha Brahmana 9.1.2.43

9.1.2.43

tā haitā amṛteṣṭakāḥ | tāuttamā upadadhātyamṛtaṃ tadasya sarvasyottamaṃ dadhāti tasmādasya sarvasyāmṛtamuttamaṃ nānyo 'dhvaryorgāyediṣṭakā vā etā vicito ha syādyadanyo 'dhvaryorgāyet

These (hymns) are the immortal bricks; he lays them down last (highest) of all:--he thereby makes immortality the highest thing of all this (universe), and hence immortality is the highest thing of all this (universe). Let none other but the Adhvaryu sing; for these (hymns) are bricks, and he (Agni, the fire-altar) would be built up in the wrong way, were any other than the Adhvaryu to sing.