Śatapatha Brāhmaṇa ("of a hundred paths") 4.4.5.19

4.4.5.19

etadādityānāmayanam | ādityānīmāni yajūṃṣītyāhuḥ sa yāvadasya vaśaḥ syādevameva cikīrṣedyadyu enamitarathā yajamānaḥ kartavai brūyāditaratho tarhi kuryādetāneva caturaḥ prayājānapabarhiṣo yajeddvāvājyabhāgau varuṇamagnīvaruṇau dvāvanuyājāvapabarhiṣau taddaśa daśākṣarā vai virāḍvirāḍvai yajñastadvirājamevaitadyajñamabhisampādayati

This is the course of the Âdityas; and these Yagus, they say, belong to the Âdityas. Let (the Adhvaryu) endeavour to perform as much of it as is his (the sacrificer's) wish. And if the sacrificer tell him to do otherwise, then he should do otherwise. He may also perform those same four fore-offerings,--omitting that to the Barhis--two butter-portions, (the oblations of cake) to Varuna and Agni-Varuna, and two after-offerings,--omitting the one to the Barhis;--this makes ten. Now the virâg consists of ten syllables, and the sacrifice is virâg: thus he makes the sacrifice to be like the virâg.