Bhagavad Gita: Chapter 2, Verse 5

गुरूनहत्वा हि महानुभावान्
श्रेयो भोक्तुं भैक्ष्यमपीह लोके ।
हत्वार्थकामांस्तु गुरूनिहैव
भुज्ज‍ीय भोगान्‍रुधिरप्रदिग्धान् ॥ २-५ ॥

gurūn ahatvā hi mahānubhāvān
śreyo bhoktuṁ bhaikṣyam apīha loke
hatvārtha-kāmāṁs tu gurūn ihaiva
bhuñjīya bhogān rudhira-pradigdhān 2.5

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.