Chandogya Upanishad 7.3

'Mind (manas) is better than speech. For as the closed fist holds two amalaka or two kola or two aksha fruits, thus does mind hold speech and name. For if a man is minded in his mind to read the sacred hymns, he reads them; if he is minded in his mind to perform any actions, he performs them; if he is minded to wish for sons and cattle, he wishes for them; if he is minded to wish for this world and the other, he wishes for them. For mind is indeed the self, mind is the world, mind is Brahman. Meditate on the mind.

'He who meditates on the mind as Brahman, is, as it were, lord and master as far as the mind reaches—he who meditates on the mind as Brahman.' 'Sir, is there something better than mind?' 'Yes, there is something better than mind.' 'Sir, tell it me.'