Chandogya Upanishad 6.7.5

6.7.5

taṃ hovāca \
yathā somya mahato 'bhyāhitasyaikam aṅgāraṃ khadyotamātraṃ pariśiṣṭaṃ taṃ tṛnair upasamādhāya prājvalayet tena tato 'pi bahu dahet \\607.5\

(4) Then he ate. So then he approached him. Then whatsoever he asked him, he answered everything. (5) To him he then said:

5. ‘Just as, my dear, one may, by covering it with straw, make a single coal of the size of a fire-fly that has been left over from a great kindled fire blaze up, and with it the fire would thereafter burn much— so, my dear, of your sixteen parts a single sixteenth part has been left over. After having been covered with food, it has blazed up. With it you now apprehend the Vedas; for, my dear, the mind consists of food, the breath consists of water, the voice consists of heat.’ Then he understood from him—yea, he understood.