Śatapatha Brāhmaṇa ("of a hundred paths") 13.4.3.4

13.4.3.4

sampraiṣyādhvaryuḥ prakramānjuhoti anvāhāryapacane vāśvasya vā padam parilikhya yatarathāsya tatrāvṛdbhavati pūrvā tveva sthitiḥ

Having called (on the masters of lute-players), the Adhvaryu performs the Prakrama oblations, either on the southern fire, or on a footprint of the horse, after drawing lines round it--whichever is the practice there; but the former is the established rule.