Shatapatha Brahmana 13.2.7.1

13.2.7.1

niyukteṣu paśuṣu prokṣaṇīradhvaryurādatte'śvam prokṣiṣyannanvārabdhe yajamāna ādhvarikaṃ yajuranudrutyāśvamedhikaṃ yajuḥ pratipadyate

When the victims have been bound (to the stakes), the Adhvaryu takes the sprinkling-water in order to sprinkle the horse. Whilst the Sacrificer holds on to him behind, he (in sprinkling the horse) runs rapidly through the formula used at the Soma-sacrifice, and then commences the one for the Asvamedha.