Śatapatha Brāhmaṇa ("of a hundred paths") 2.4.2.17

2.4.2.17

atha sakṛdācinnānyupamūlaṃ dināni bhavanti | agramiva vai devānām madhyamiva manuṣyāṇām mūlamiva pitṝṇāṃ tasmādupamūlaṃ dināni bhavanti sakṛdācinnāni bhavanti sakṛdu hyevaparāñcaḥ pitarastasmātsakṛdācinnāni bhavanti

Now those (stalks of sacrificial grass) are severed with one stroke, and cut off near the root;--the top belongs to the gods, the middle part to men, and the root-part to the fathers: therefore they are cut off near the root. And with one stroke they are severed, because the fathers have passed away once for all.