Śatapatha Brāhmaṇa ("of a hundred paths") 10.1.3.2

10.1.3.2

tasya ha prajāpateḥ ardhameva martyamāsīdardhamamṛtaṃ tadyadasya martyamāsīttena mṛtyorabibhetsa bibhyādimām prāviśaddvayam bhūtvā mṛccāpaśca

Now, one half of that Pragâpati was mortal, and the other half immortal: with that part of him which was mortal he was afraid of death; and, being afraid, he became twofold, clay and water, and entered this (earth).