KAṬHA UPANISHAD · 1.3.13

यच्छेद्वाङ्मनसी प्राज्ञस्तद्यच्छेज्ज्ञान आत्मनि ।
ज्ञानमात्मनि महति नियच्छेत्तद्यच्छेच्छान्त आत्मनि ॥ १३॥

yacchedvāṅmanasī prājñastadyacchejjñāna ātmani ।
jñānamātmani mahati niyacchettadyacchecchānta ātmani ॥ 13॥

Let the wise man restrain speech in his mind and mind in his self of knowledge, and knowledge in the Great Self, and that again let him restrain in the Self that is at peace.