Shankaracharya says in his work Upadesa Sahasri (A Thousand Teachings, Swami Jagadananda translator) verse 6:
...He [the teacher, the guru] should first of all teach the Sruti texts [vedic] establishing the oneness of the Self with Brahman such as "My child, in the beginning it (the universe) was Existence only, one alone without a second" (Chhandogya Upanishad 6.2.1), Where one sees nothing else (Chh. U. 7.24.1), "All this is bu the Self" (Chh. U. 7.25.2), "In the beginning all this was but the one Self" (Aitareya Upanishad 1.1.1), and "All this is verily Brahman." (Chh. U. 3.14.1)
and further in verses 7 and 8:
After teaching these he should teach the definition of Brahman through such Sruti texts as "The Self devoid of sins" (Chh U. 8.7.1), "The Brahman that is immediate and direct" (Brihadaranyaka Upanishad 3.4.1), "That which is beyond hunger and thirst" (Br. U. 3.5.1), "Not-this, not-this" (Br. U. 2.3.6), "Neither gross nor subtle" (Br. U. 3.8.8), "This Self is not-this" (Br. U. 3.9.26), ""It is the Seer Itself unseen" (Br. U. 3.8.11), "Knowledge-Bliss" (Br. U. sloka 7, under 3.9.27), "Existence-Knowledge-Infinite" (Taittareya Upanishad 2.1), "Imperceptible bodiless" (Tai. U. 2.7), "That great unborn Self" (Br. U. 4.4.22), "Without the vital forces and the mind" (Mundaka Upanishad 2.1.2), "Unborn, comprising the interior and exterior" (Mu. U. 2.1.2), "Consisting of Knowledge only" (Br. U. 2.4.12), "Without interior or exterior" (Br. U. 2.5.19), "It is verily beyond what is known as also what is unknown" (Kena Upansihad 1.3) and "Called Akasha (the self-effulgent One)" (Chh. U. 8.14.1), and also through such Smriti texts as the following: "It is neither born nor dies" (Bh. Gita 2.20), "It is not affected by anybody's sins" (Bh. Gita 5.15), "Just as air is always in the ether" (Bh. Gita 9.6), "The individual Self should be regarded as the universal one" (Bh. Gita 13.2), "It is called neither existent nor non-existent" (Bh. Gita 13.12), "As the Self is beginningless and devoid of all qualities" (Bh. Gita 13.31), "The same in all beings" (Bh. Gita 13.27), and "The Supreme Being is different" (Bh. Gita 15.17); --all these support the definition given by the Srutis and prove the innermost Self is beyond transmigratory existence and that it is not different from Brahman, the all-comprehensive principle.
Chandradhar Sharma gives some other examples of references in the Rig Veda is his section on the vedas in his book here - https://archive.org/details/IndianPhilosophyACriticalSurvey. He also goes into the development/evolution of the concept of maya. You might also like reading the first 2 chapters of the Brahma Sutras (https://www.wisdomlib.org/hinduism/book/brahma-sutras). It deals with the definition of Brahman. Vireswarananda goes inot the various definitions in his Introduction and the second section - Adhyasa - deals with Shankara's concept of Maya.