Bhagavad Gita
Chapter 6 · Verse 6
बन्धुरात्मात्मनस्तस्य येनात्मैवात्मना जितः। अनात्मनस्तु शत्रुत्वे वर्तेतात्मैव शत्रुवत्॥
bandhur ātmātmanas tasya yenātmaivātmanā jitaḥ, anātmanas tu śatrutve vartetātmaiva śatru-vat
The same mind can work for you or against you. The difference is whether it's been trained or left to run unchecked.
A mastered mind is your best friend. An unmastered one, your worst enemy.