Katha Upanishad 2.15 – 17

 Yama Replied :- That goal which all the Vedas glorify, desiring which people practice Brahmacharya (A Life of Continence, Peace and Selfless Service), that goal, I tell thee briefly – It is Aum. This word is indeed Brahman. This word is indeed Supreme. He who knows this word attains the peace he desires. This is the best support; This is the highest support. He who knows this support is glorified in the world of Brahman.”