Bhagavad Gita 2.5

गुरूनहत्वा हि महानुभावान् श्रेयो भोक्तुं भक्ष्यमपीह लोके |
हत्वार्थमांस्तु गुरूनिहैव भुञ्जिय भोगान रुधिरप्रदिग्धन

Translation

It would be better to live in this world by begging, than to enjoy life by killing these noble elders, who are my teachers. If we kill them, the wealth and pleasures we enjoy will be tainted with blood.