Shatapatha Brahmana 13.1.4

Pragâpati poured forth the life-sap of the horse (asva-medha). When poured forth, it went straight away from him and spread itself over the regions. The gods went in quest of it. By means of offerings (ishti) they followed it up, by offerings they searched for it, and by offerings they found it. And when he performs ishtis, the Sacrificer thereby searches for the horse (asva) meet for sacrifice (medhya).

They (the ishtis) belong to Savitri; for Savitri is this (earth): if any one hides himself thereon, if any one goes elsewhere, it is on this (earth) that they find him; for no one (creature), whether walking erect or horizontally (like an animal), is able to go beyond it. Their belonging to Savitri thus is in order to find the horse.

Concerning this they say, 'Surely the horse disappears when it goes straight away; for they do not turn (drive) it back.' Now when he performs the Dhriti offerings in the evening--dhriti (keeping) meaning peaceful dwelling, and the night also meaning peaceful dwelling--it is by means of peaceful dwelling that he keeps it; whence both men and beasts rest peacefully at night. And when he performs offerings in the morning, he seeks that (horse); whence it is in daytime that one goes to seek for what is lost. And again when he offers the Dhritis in the evening, and the (Savitri) ishtis in the morning, it is security of possession the Sacrificer thereby brings about, whence security of possession is brought about for the subjects where this sacrifice is performed.