Srimad-Bhagavatam: Canto 6 - Chapter 1 - Verse 8
Sanskrit:
तुर्यं छेदविरोहेण वरेण जगृहुर्द्रुमा: ।तेषां निर्यासरूपेण ब्रह्महत्या प्रदृश्यते ॥ ८ ॥
ITRANS:
turyaṁ cheda-viroheṇavareṇa jagṛhur drumāḥteṣāṁ niryāsa-rūpeṇabrahma-hatyā pradṛśyate
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.]