Bhagavad Gita 13.5ऋषिभिर्बहुधा गीतं छन्दोभिर्विविधैः पृथक्।ब्रह्मसूत्रपदैश्चैव हेतुमद्भिर्विनिश्िचतैः ॥

Many sages (such as Vasishtha) have talked about it (the true nature of the field and its knower) since ancient times. The ancient hymns, such as the Rig Veda, have explained this in various ways.The word Brahma Sutras refers to the Vedanta Sutras written by Vyasa or Badarayanacharya in order to reconcile the mutually contradictory passages in the Upanishads. A study of the Brahma Sutras is very necessary in order to comprehend the esoteric significance of the Upanishads. The Braham Sutras are also known by the name Sariraka Sutras because fifteen Sutras in the third Pada of the second chapter deal with the Sarira or Kshetra (body).The true nature of the field and its knower has also been taught in the Brahma Sutras which deal with Brahman such as Atmanyevopasita (only as the Self, let a man meditate on It.) (Brihadaranyaka Upanishad, I.4.7).They are full of reasoning, convincing and decisive. There is no doubt in the words or passages that treat of Brahman.