Brihadâranyaka Upanishad: V, 6

SIXTH BRÂHMANA. 1. That person, under the form of mind (manas), being light indeed 2, 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. 192:2 Bhâhsatya must be taken as one word, as the commentator says, bhâ eva satyam sadbhâvah svarûpam yasya so 'yam bhâhsatyo bhâsvarah.