Shatapatha Brahmana 13.5.1.16

13.5.1.16

atha purā bahiṣpavamānāt aśvaṃ niktvodānayanti tena pāvamānāya sarpanti tasyoktam brāhmaṇaṃ stute bahiṣpavamāne'śvamāstāvamākramayanti sa yadyava vā jighredvi vā vartet samṛddho me yajña iti ha vidyāttamupākṛtyādhvaryurāha hotarabhiṣṭuhīti tamekādaśabhirhotābhiṣṭauti

Now, prior to the (chanting of the) Bahishpavamâna, they (the assistants of the Adhvaryu) bring up the horse, after cleansing it; and with it they glide along for the Pavamâna: the mystic import of this has been explained. When the Bahishpavamâna has been chanted, they make the horse step on the place of chanting: if it sniffs, or turns away, let him know that his sacrifice is successful. Having led up that (horse), the Adhvaryu says, 'Hotri, sing praises!' and the Hotri sings its praises with eleven (verses, Rig-veda I, 163, 1-11)--