Śatapatha Brāhmaṇa ("of a hundred paths") 1.3.1.26

1.3.1.26

so 'vekṣate | satyamṛ vai cakṣuḥ satyaṃ hi vai cakṣustasmādyadidānīṃ dvau vivadamānāveyātāmahamadarśamahamaśrauṣamiti ya eva brūyādahamadarśamiti tasmā eva śraddadhyāma tatsatyenaivaitatsamardhayati

He then looks down on the butter. Here now some make the sacrificer look down. Yâgñavalkya, however, said in reference to this point,--'Why do not (the sacrificers) themselves become (act as) Adhvaryu priests? and why do not they (the sacrificers) themselves recite when far higher blessings are prayed for? How can these (people) possibly have faith in this? Whatever blessing the officiating priests invoke during the sacrifice that is for the benefit of the sacrificer alone.' The Adhvaryu should accordingly look down on it.