That which is the subtle essence — in It all that exists has its Self. That is the truth. That is the Self. That thou art, O Svetaketu. ("Tat tvam asi" — one of the four mahavakyas.)
Chandogya Upanishad
छान्दोग्योपनिषद्
मूल श्लोकः
स य एषोऽणिमैतदात्म्यमिदꣳ सर्वं तत्सत्यꣳ स आत्मा तत्त्वमसि श्वेतकेतो॥
sa ya eṣo 'ṇimaitad-ātmyam idaṁ sarvaṁ tat satyaṁ sa ātmā tat tvam asi śvetaketo
ChU 6.8007