SB 6.9.8

turyaṁ cheda-viroheṇa
 vareṇa jagṛhur drumāḥ
teṣāṁ niryāsa-rūpeṇa
 brahma-hatyā pradṛśyate
Word for word: 
turyam — one fourth; cheda — although being cut; viroheṇa — of growing again; vareṇa — because of the benediction; jagṛhuḥ — accepted; drumāḥ — the trees; teṣām — of them; niryāsa-rūpeṇa — by the liquid oozing from the trees; brahma-hatyā — the reaction for killing a brāhmaṇa; pradṛśyate — is visible.
Translation: 
In return for Indra’s benediction that their branches and twigs would grow back when trimmed, the trees accepted one fourth of the reactions for killing a brāhmaṇa. These reactions are visible in the flowing of sap from trees. [Therefore one is forbidden to drink this sap.]