Śatapatha Brāhmaṇa ("of a hundred paths") 3.1.3.3

3.1.3.3

aṣṭau ha vai putrā aditeḥ | yāṃstvetaddevā ādityā ityācakṣate sapta haiva te 'vikṛtaṃ hāṣṭamaṃ janayāṃ cakāra mārtāṇḍaṃ saṃdegho haivāsa yāvānevordhvastāvāṃstiryaṅ puruṣasammita ityu haika āhuḥ

Now Aditi had eight sons. But those that are called 'the gods, sons of Aditi,' were only seven, for the eighth, Mârtânda, she brought forth unformed: it was a mere lump of bodily matter, as broad as it was high. Some, however, say that he was of the size of a man.