Shatapatha Brahmana 1.7.2.9

1.7.2.9

sa vai yāvanmātramivaivāvadyet mānuṣaṃ ha kuryādyanmahadavadyedvyṛddhaṃ vai tadyajñasya yanmānuṣaṃ nedvyṛddhaṃ yajñe karavāṇīti tasmādyāvanmātramivaivāvadyet

Let him cut off only a moderate quantity; for were he to cut off a large quantity, he would make it human; and what is human is inauspicious at the sacrifice. Let him therefore cut off only a moderate quantity, lest he should do what is inauspicious at the sacrifice.