Śatapatha Brāhmaṇa ("of a hundred paths") 11.4.2.20

11.4.2.20

sa hovāca upa tvāyānīti sa hovācātra vāva khalvarhasi yo naḥ saṃvatsare'dhvaryurabhūranupetāyaiva ta etadbravāṇīti tasmā u haitadeva srucorādānamuvāca yadetadvyākhyām tasmādevaṃvidamevādhvaryuṃ kurvītanānevaṃvidam

He said, 'Let me become thy pupil.' He answered, 'Even now, indeed, art thou worthy (of being instructed), who hast been our Adhvaryu for a year: I will teach thee this even without thy becoming my pupil.' And so, indeed, he taught him that taking up of the two spoons as we have here explained it: hence one ought only to make one who knows this his Adhvaryu, and not one who does not know it.