Śatapatha Brāhmaṇa ("of a hundred paths") 8.6.2.17

8.6.2.17

pratiṣṭhā dvipadāḥ | tā yaddvipadā bhavanti dvandvaṃ hi pratiṣṭhā tisro bhavanti trivṛddhi pratiṣṭhā paścādupadadhāti paścādhīyam pratiṣṭhā

The Dvipadâs are the feet (the stand);--and as to its being Dvipadâs (verses of two feet), it is because the feet are a pair. There are three (such verses), for a stand (tripod) is threefold. He lays them down at the back, for the feet are at the back (of the body).