Chapter 6 · verse 6 · gita.yoga.org

Bhagavad Gita 6.6

बन्धुरात्माऽऽत्मनस्तस्य येनात्मैवात्मना जितः। अनात्मनस्तु शत्रुत्वे वर्तेतात्मैव शत्रुवत्

The Self is the friend of the self of him by whom the Self has been conquered; but to the unconquered self, this Self stands in the position of an enemy, like an external foe.