Bg 6.6

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

Word for word: 
bandhuh — friend; atma — the mind; atmanah — of the living entity; tasya — of him; yena — by whom; atma — the mind; eva — certainly; atmana — by the living entity; jitah — conquered; anatmanah — of one who has failed to control the mind; tu — but; satrutve — because of enmity; varteta — remains; atma eva — the very mind; satru-vat — as an enemy.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
For him who has conquered the mind, the mind is the best of friends; but for one who has failed to do so, his mind will remain the greatest enemy.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The purpose of practicing eightfold yoga is to control the mind in order to make it a friend in discharging the human mission. Unless the mind is controlled, the practice of yoga (for show) is simply a waste of time. One who cannot control his mind lives always with the greatest enemy, and thus his life and its mission are spoiled. The constitutional position of the living entity is to carry out the order of the superior. As long as one’s mind remains an unconquered enemy, one has to serve the dictations of lust, anger, avarice, illusion, etc. But when the mind is conquered, one voluntarily agrees to abide by the dictation of the Personality of Godhead, who is situated within the heart of everyone as Paramatma. Real yoga practice entails meeting the Paramatma within the heart and then following His dictation. For one who takes to Krishna consciousness directly, perfect surrender to the dictation of the Lord follows automatically.