Bhagavad Gita: Chapter 2, Verse 66

नास्ति बुद्धिरयुक्तस्य न चायुक्तस्य भावना ।
न चाभावयतः शान्तिरशान्तस्य कुतः सुखम् ॥ २-६६ ॥

nāsti buddhir ayuktasya na cāyuktasya bhāvanā
na cābhāvayataḥ śāntir aśāntasya kutaḥ sukham 2.66

One who is not connected with the Self can have neither transcendental intelligence nor a steady mind, without which there is no possibility of peace. To the man who has no peace, how can there be happiness?