Garudapurana 1.48.81

1.48.81

ājyaṃ tayoḥ sahakāri tatpradhānaṃ yadaṅka(kṣa)yoḥ /
paruṣasuktaṃ pūrveṇaiva rudracaiva tu dakṣiṇe //

Clarified butter is the auxiliary to both; when both are exhausted, it is principal; / with the Puruṣasūkta in the east and with the Rudra hymn in the south;