Shatapatha Brahmana 8.7.4.18

8.7.4.18

amṛtameva saptamī citiḥ tāmuttamāmupadadhātyamṛtaṃ tadasya sarvasyottamaṃ dadhāti tasmādasya sarvasyāmṛtamuttamaṃ tasmāddevā anantarhitāstasmādu te 'mṛtā ityadhidevatam

The seventh layer is immortality,--that is the last (layer) he lays down, and thus bestows immortality as the highest thing of all this (universe): therefore immortality is the highest thing of all this (universe); therefore the gods are not separated therefrom; and therefore they are immortal. Thus much as to the deity.