Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.2.17

7.3.2.17

athāto 'dhirohaṇasyaiva | taṃ haike purastātpratyañcamadhirohanti paścādvā prāñcaṃ na tathā kuryātpaśureṣa yadagniryo vai paśum purastātpratyañcamadhirohati viṣāṇābhyāṃ taṃ hantyatha yaḥ paścātprāñcam padbhyāṃ tamātmanevainamārohedyaṃ vā ātmanā paśumārohanti sa pārayati sa na hinastyuttarato yaṃ hi kaṃ ca paśumārohantyuttarata evainammārohantyāruhyāgnimauttaravedikaṃ karma kṛtvātmannagniṃ gṛhṇīta ātmannagniṃ gṛhītvā satyaṃ sāma gāyati puṣkaraparṇamupadadhāti tasyātaḥ

Now, then, as to the mounting (of the altar). Now some mount it from the front (east) towards the back, or from the back towards the front: let him not do so; for that Agni (the fire-altar) is an animal; and if one mounts an animal (ox) from the front towards the back, it strikes him with its horns; and if he mounts it from the back towards the front, it does so with its feet. Let him mount it only by the middle body; for the animal which people mount by the (middle) body, carries them forward, and does not hurt them. From the left (north) side (he should mount it), for any animal which people mount they mount from the left side. By mounting the (body of the) altar from the left side, and performing the work connected with the Uttara-vedi, he takes hold of Agni in the (middle) body (or, into himself); and having taken Agni into his own self, he sings the 'true hymn.' He puts a lotus-leaf on (the altar): thereof further on.