Shatapatha Brahmana 1.9.2.25

1.9.2.25

atha samiṣṭayajurjuhoti | prāṅne yajño 'nusaṃtiṣṭhātā ityatha yaddhutvā samiṣṭayajuḥ patnīḥ saṃyājayetpratyaṅṅu haivāsyaiṣa yajñaḥ saṃtiṣṭheta tasmādvā etarhi samiṣṭayajurjuhoti prāṅne yajño 'nusaṃtiṣṭhātā iti

He (the Adhvaryu) now makes the samishtayagus-oblation, thinking, 'In the east my sacrifice shall be completed!' Were he to perform the samishtayagus-oblation first and then the patnîsamyâgas, that sacrifice of his would be completed in the west (behind the sacrificer): hence he makes the samishtayagus-oblation at this particular time, thinking, 'In the east my sacrifice shall be completed!'