Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.1.1

2.5.1.1

prajāpatirha vā idamagra eka evāsa | sa aikṣata kathaṃ nu prajāyeyeti so 'śrāmyatsa tapo 'tapyata sa prajā asṛjata tā asya prajāḥ sṛṣṭāḥ parābabhūvustānīmāni vayāṃsi puruṣo vai prajāpaternediṣṭhaṃ dvipādvā ayam puruṣastasmāddvipādo vayāṃsi

Verily, in the beginning, Pragâpati alone existed here. He thought within himself, 'How can I be propagated?' He toiled and practised austerities. He created living beings. The living beings created by him passed away: they are those birds. Now man is the nearest to Pragâpati; and man is two-footed: hence birds are two-footed.