Shatapatha Brahmana 9.5.1.10

9.5.1.10

tatprātarabhipadya | abhiṣutyāgnau juhoti tadagnāvamṛtaṃ dadhātyatha bhakṣayati tadātmannamṛtaṃ dhatte so 'mṛto bhavatyetadvai manuṣyasyāmṛtatvaṃ yatsarvamāyureti tatho hānenātmanā sarvamāyureti

And, on the morrow, having laid hold of it, and pressed it, he offers (of it) in the fire, and thereby bestows immortality on Agni. He then drinks (of it), and thereby bestows immortality on his own self, and becomes immortal; for this, assuredly, is immortality to man when he attains the whole (perfect) life: and so, in truth, he attains the whole life by this self of his.