Bhagavad Gita 2.66

नास्ति बुद्धिरयुक्तस्य न चाक्तरस्य भावना |
न चाभावयत्: शान्तिरशनान्तस्य कुत: सुखम्

Translation

But an undisciplined person, who has not controlled the mind and senses, can neither have a resolute intellect nor steady contemplation on God. For one who never unites the mind with God there is no peace; and how can one who lacks peace be happy?