6:6

बन्धुरात्मात्मनस्तस्य येनात्मैवात्मना जितः ।
अनात्मनस्तु शत्रुत्वे वर्तेतात्मैव शत्रुवत् ॥६- ६॥

Transliteration

bandhuraatmaatmanastasya yenaatmaiwaatmanaa jitaH ।

anaatmanastu shatrutwe wartetaatmaiwa shatruwat ॥६- ६॥

The Self [and] self are friends whose Self has conquered self.
The self remaining in enmity is like the not-Self enemy.

The mind is the friendxe "friend" of those who have control over it, and the mind acts like an enemy for those who do not control it.

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.