BṚHADĀRAṆYAKA UPANISHAD · 5.6.1

मनोमयो ऽयं पुरुषो भाःसत्यः ।
तस्मिन्न् अन्तर् हृदये यथा व्रीहिर् वा यवो वा ।
स एष सर्वस्येशानः सर्वस्याधिपतिः सर्वम् इदं प्रशास्ति यद् इदं किञ्च ॥

manomayo ’yaṃ puruṣo bhāḥsatyaḥ ।
tasminn antar hṛdaye yathā vrīhir vā yavo vā ।
sa eṣa sarvasyeśānaḥ sarvasyādhipatiḥ sarvam idaṃ praśāsti yad idaṃ kiñca ॥

That person, under the form of mind (manas), being light indeed, is within the heart, small like a grain of rice or barley. He is the ruler of all, the lord of all--he rules all this, whatsoever exists.

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)