Sadguru Swami Gangeshwaranandji Maharaj said:

The gist of all the Vedas is contained in the word SOHAM

Shukla Yajur Veda (Kaarva Shukla 40-16)

These Vakya (sentences) are described in the Vedas and the Upanishads as 4 MAHA VAKYAS

Prajnanam Brahma (प्रज्ञानम् ब्रह्म)

This is the first Maha Vakya from the Rig Veda and Aetriya Upanishad (3-3)

Aham Brahma Asmi (अहम् ब्रह्म अस्मि)

The second Maha Vakya is from the Yajur Veda (Kaarva Shakha)and from the Brahdaranyaka Upanishad (1-4-10)

Tat Tvam Asi (तत् त्वम् असि)

This third Maha Vakya is from the Sam Veda and Chandogya Upanishad (6-8-7)

 Ayam Atma Brahma (अयम् आत्मा ब्रह्म)

This 4th Maha Vakya is from the Atharva Veda and Mandukya Upanishad (mantra 2)