Shatapatha Brahmana 10.4.1.19

10.4.1.19

tasmā etasmai saptadaśāya prajāpataye etatsaptadaśamannaṃ samaskurvanya eṣa saumyo'dhvaro'tha yā asya tāḥ ṣoḍaśa kalā ete te ṣoḍaśartvijastasmānna saptadaśamṛtvijaṃ kurvīta nedatirecayānītyatha ya evātra raso yā āhutayo hūyante tadeva saptadaśamannam

Now for that seventeenfold Pragâpati they prepared this seventeenfold food, the Soma-sacrifice: those sixteen digits of his are these sixteen officiating priests,--one should not, therefore, take a seventeenth priest lest one should do what is excessive;--and what vital sap there is here--the oblations that are offered--that is the seventeenfold food.