Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.3.21

3.2.3.21

tadāhuḥ | yadeva prāyaṇīye kriyeta tadudayanīye kriyeta yadeva prāyaṇīyasya barhirbhavati tadudayanīyasya barhirbhavatīti tadapoddhṛtya nidadhāti tāṃ sthālīṃ sakṣāmakarṣām pramṛjya mekṣaṇaṃ nidadhāti ya eva prāyaṇīyasyartvijo bhavanti ta udayanīyasyartvijo bhavanti tadyadetatsamānaṃ yajñe kriyate tena bāhū sadṛśau tena sarūpau

Now, they say that whatever is done at the Prâyanîya should be done at the Udayanîya, and the barhis (grass-covering of the altar), which is used at the Prâyanîya, is also used at the Udayanîya: he lays it aside, after removing it (from the altar). The pot (in which the rice-pap was cooked) he puts aside with the parched remains of dough, and (so he does) the pot-ladle after wiping it. And the priests who officiate during the Prâyanîya, officiate also at the Udayanîya. And because of this identical performance at the sacrifice the two arms are alike and of the same shape.