Bhagavad Gita: For him who has conquered the mind, the mind is the best of friends (Dhyana-yoga (Text 06, Ch 6))

Sri-bhagavan uvaca

bandhur atmatmanas tasya
yenatmaivatmana jitah
anatmanas tu satrutve
vartetatmaiva satru-vat

Meaning: For him who has conquered the mind, the mind is the  best friends; but for one who has failed to do so, his mind will remain the greatest enemy.

***
One who cannot control his mind lives always with the greatest  enemy, and thus his life and its mission are spoiled

Bhagavad Gita: For him who has conquered the mind, the mind is the best of friends (Dhyana-yoga (Text 06, Ch 6))