Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.3.23

3.2.3.23

sa vai pañca prāyaṇīye devatā yajati | pañcodayanīye tasmātpañcetthādaṅgulayaḥ pañcetthāttacamyvantam bhavati na patnīḥ saṃyājayanti pūrvārdhaṃ vā anvātmano bāhū pūrvārdhamevaitadyajñasyābhisaṃskaroti tasmācamyvantam bhavati na patnīḥ saṃyājayanti

To five deities he offers at the Prâyanîya, and to five at the Udayanîya: hence there are five fingers here and five there. This (Prâyanîya offering) ends with the Samyu. They perform no Patnîsamyâgas. For the arms are on the fore-part of the body, and the fore-part of the sacrifice he perfects by this (opening ceremony). This is why it ends with the Samyu, and why no Patnîsamyâgas are performed.