Bhagavad Gita · 2.5

गुरूनहत्वा हि महानुभावान्श्रेयो भोक्तुं भैक्ष्यमपीह लोके।हत्वार्थकामांस्तु गुरूनिहैवभुञ्जीय भोगान् रुधिरप्रदिग्धान्॥

gurūn ahatvā hi mahānubhāvān śhreyo bhoktuṁ bhaikṣhyam apīha loke hatvārtha-kāmāṁs tu gurūn ihaiva bhuñjīya bhogān rudhira-pradigdhān

It would be better to live in this world by begging than to slay these great souls who are my teachers. If I kill them, all my enjoyments of wealth and desires will be tainted with blood.

— Bhagavad Gita 2.5

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)