mano durnigraham calam
abhyasena tu kaunteya
vairagyena ca grhyate
(Dhyana-yoga, Text 35 Chapter 6)
Meaning: Lord Sri Krsna said: O mightly-armed son of kunti, it is undoubtedly very difficult to curb the restless mind, but it is possible by suitable practice and by detachment.
Bhagavad Gita: It is possible by suitable practice and by detachment to control the mind (Dhyana-Yoga, Text 35 Chapter 6)