Shatapatha Brahmana 9.4.1.15

9.4.1.15

yadveva rathaśīrṣe juhoti | asau vā āditya eṣa ratha etadvai tadrūpaṃ kṛtvā prajāpatiretāni mithunāni parigatyātmannadhattātmannakuruta tathaivainānyayametatparigatyātmandhatta ātmankuruta upari dhāryamāṇa upari hi sa ya etāni mithunāni parigatyātmannadhattātmannakuruta samānena mantreṇa samāno hi sa ya etāni mithunāni parigatyātmannadhattātmannakuruta sarvataḥ parihāraṃ sarvato hi sa ya etāni mithunāni parigatyātmannadhattātmannakuruta

And, again, as to why he makes offering on the head of the chariot;--it is because this chariot is yonder sun; for it was by assuming that form that Pragâpati enclosed those couples, and took them to himself, and made them his own; and in like manner does this (Sacrificer) thereby enclose them, and take them to himself, and make them his own. Whilst it (the chariot-head) is held above (the fire, he offers), for above (others) was he who, enclosing those couples, took them to himself, and made them his own;--and with the same formula, for one and the same is he who, by enclosing those couples, took them to himself, and made them his own.