Śatapatha Brāhmaṇa ("of a hundred paths") 10.4.3.9

10.4.3.9

sa mṛtyurdevānabravīt itthameva sarve manuṣyā amṛtā bhaviṣyantyatha ko mahyam bhāgo bhaviṣyatīti te hocurnāto'paraḥ kaścana saha śarīreṇāmṛto'sadyadaiva tvametam bhāgaṃ harāsā atha vyāvṛtya śarīreṇāmṛto'sadyo'mṛto'sadvidyayā vā karmaṇā veti yadvai tadabruvanvidyayā vā karmaṇā vetyeṣā haiva sā vidyā yadagniretadu haiva tatkarma yadagniḥ

Death spake unto the gods, 'Surely, on this wise all men will become immortal, and what share will then be mine?' They spake, 'Henceforward no one shall be immortal with the body: only when thou shalt have taken that (body) as thy share, he who is to become immortal either through knowledge, or through holy work, shall become immortal after separating from the body.' Now when they said, 'either through knowledge or through holy work,' it is this fire-altar that is the knowledge, and this fire-altar that is the holy work.