Varahashrautasutra 2.2.2.17

2.2.2.17

parameṣṭhī tvā sādayatu divaḥ pṛṣṭhe vyacasvatīmiti svayamātṛṇṇāmadhvaryuḥ prothadaśva iti vikarṇī m̐ śarkarāṃ pratiprasthātā yugapadupadhattaḥ

With ‘May Parameṣṭhin seat you on the back of heaven, spacious’, the adhvaryu [places] the naturally perforated [brick]; with ‘Snorting horse’, the pratiprasthātṛ [places] the vikarṇī gravel; they both place [them] simultaneously.