As I discuss in this question, by far the most popular school of Hindu philosophy is the Vedanta school, which bases its tenets on the doctrines laid out in the Brahma Sutras, a work by the sage Vyasa which summarizes and systematizes the philosophical teachings of the Upanishads. You can read the Brahma Sutras here. In any case, Adhyaya 3 Pada 3 of the Brahma Sutras describes the Brahma Vidyas, 32 lessons found in the various Upanishads which can each lead you to Brahman if you meditate upon them. You can see the full list of 32 Vidyas here.
Now as I discuss in this question, one of these 32 Brahma Vidyas is known as the Madhu Vidya, or "honey wisdom". It's found in the Brihadaranyaka and Chandogya Upanishads, it was taught by the sage Dadhichi to the two Ashwini Kumaras while Dadhichi had a horse head in the place of his human head. In any case, in this chapter of the Brihadaranyaka Upanishad, four Vedic verses are given which allude to the Madhu Vidya and/or the story of how it was taught:
Verily Dadhyak Âtharvana proclaimed this honey (the madhu-vidyâ) to the two Asvins, and a Rishi, seeing this, said (Rv. I, 116, 12): 'O ye two heroes (Asvins), I make manifest that fearful deed of yours (which you performed) for the sake of gain, like as thunder makes manifest the rain. The honey (madhu-vidyâ) which Dadhyak Âtharvana proclaimed to you through the head of a horse,'
Verily Dadhyak Âtharvana proclaimed this honey to the two Asvins, and a Rishi, seeing this, said (Rv. I, 117, 22): 'O Asvins, you fixed a horse's head on Âtharvana Dadhyak, and he, wishing to be true (to his promise), proclaimed to you the honey, both that of Tvashtri and that which is to be your secret, O ye strong ones.'
Verily Dadhyak Âtharvana proclaimed this honey to the two Asvins, and a Rishi, seeing this, said: 'He (the Lord) made bodies with two feet, he made bodies with four feet. Having first become a bird, he entered the bodies as purusha (as the person).' This very purusha is in all bodies the purisaya, i.e. he who lies in the body (and is therefore called purusha). There is nothing that is not covered by him, nothing that is not filled by him.
Verily Dadhyak Âtharvana proclaimed this honey to the two Asvins, and a Rishi, seeing this, said (Rv. VI, 47, 18): 'He (the Lord) became like unto every form, and this is meant to reveal the (true) form of him (the Âtman). Indra (the Lord) appears multiform through the Mâyâs (appearances), for his horses (senses) are yoked, hundreds and ten.'
The quotes in verses 16, 17, and 19 are all cited by the translators as verses from the Rig Veda. But the quote in verse 18 is not given a citation: "He (the Lord) made bodies with two feet, he made bodies with four feet. Having first become a bird, he entered the bodies as purusha (as the person)." So my question is, where in the Vedas is this quote found?
If it helps, here is the verse in Sanskrit:
puraścakre dvipadaḥ puraścakre catuṣpadaḥ ।
puraḥ sa pakṣī bhūtvā puraḥ puruṣa āviśaditi ।
Is this verse from the Rig Veda just like the other three verses, or is it from somewhere else?