The Upanishads, the Bhagavadgita and the Brahma Sutras constitute the basis of Vedanta. All schools of Vedanta propound their philosophy by interpreting these texts, collectively called the Prasthanatrayi, literally, three sources.
The Brahma Sutras attempted to synthesize the teachings of the Upanishads. The diversity in the teaching of the Upanishads necessitated the systematization of these teachings. This was likely done in many ways in ancient India, but the only surviving version of this synthesis is the Brahma Sutras of Badarayana.
Badarayana (IAST Bādarāyaṇa; Devanāgari बादरायण) was an Indian philosopher about whom almost no personal details are reliably known. He wrote the foundational philosophical treatise Vedanta school of philosophy.
Badarayana has been variously dated. The current scholarly consensus dates him to the middle of the first century CE
Badarayana is regarded as having written the basic text of the Vedanta system, the Vedāntasūtra a.k.a. Brahmasūtra. He is thus considered the founder of the Vedānta system of philosophy.
All major Vedantic teachers, including Shankara, Bhaskara, Ramanuja, Nimbarka, Vallabha and Madhva, have composed commentaries not only on the Upanishads and Brahma Sutras, but also on the Bhagavad Gita. The Bhagavad Gita, due to its syncretism of Samkhya, Yoga, and Upanishadic thought, has played a major role in Vedantic thought.