Bhagwad Gita
Chapter 6, Verse 6
बन्धुरात्माऽऽत्मनस्तस्य येनात्मैवात्मना जितः।
अनात्मनस्तु शत्रुत्वे वर्तेतात्मैव शत्रुवत्।।6.6।।
For him who has conquered the mind, the mind is the best of friends; but for one who has failed to do so, his very mind will be the greatest enemy.