Śatapatha Brāhmaṇa ("of a hundred paths") 9.5.1.8

9.5.1.8

tadyattadamṛtaṃ somaḥ saḥ | tadadyāpi yajamānaḥ śrameṇa tapasānvicati sa dīkṣitvā payovrato bhavatyetadvai tapo yo dīkṣitvā payovrato 'sattasya ghoṣamāśṛṇotītyahe kraya iti

Now that same nectar of immortality is Soma. And even to this day the Sacrificer seeks for it by toil and penance; having become initiated he lives on fast-milk; for penance it is when, after being initiated, one lives on fast-milk; he hears the sound of it, saying, 'On such and such a day the buying (will take place)!'