Śatapatha Brāhmaṇa ("of a hundred paths") 3.9.4.22

3.9.4.22

atha yasmātsomo nāma | yatra vā eṣo 'gre devānāṃ havirbabhūva taddhekṣāṃ cakre maiva sarveṇevātmanā devānāṃ havirbhūvamiti tasya yā juṣṭatamā tanūrāsa tāmapanidadhe tadvai devā aspṛṇvata te hocurupaivaitām pravṛhasva sahaiva na etayā haviredhīti tāṃ dūra ivopaprāvṛhata svā vai ma eṣeti tasmātsomo nāma

Now as to why he is called Soma. When he first became sacrificial food for the gods, he thought within him, 'I must not become, sacrificial food for the gods with my whole self!' That form of his which was most pleasing he accordingly put aside. Thereupon the gods were victorious; they said, 'Draw that unto thee, for therewith shalt thou become our food!' He drew it to him even from afar, saying, verily, that is mine own (svâ me): hence he was called Soma.