Brihadaranyaka Upanishad

बृहदारण्यकोपनिषद्

The largest Upanishad and among the oldest, from the Shukla Yajurveda. Contains the brilliant teachings of Yajnavalkya — the doctrine of Neti Neti (not this, not this), dialogues with Gargi and Maitreyi, the nature of dreams, karma and rebirth, and the Mahavakya: Aham Brahmasmi — I am Brahman.