In the Brahmasūtra bhāṣya for the very first sutra, Shankara says:
na — not; hi — indeed; acyutam — the infallible Supreme Personality of Godhead; prīṇayataḥ — satisfying; bahu — much; āyāsaḥ — endeavor; asura–ātma–jāḥ — O sons of demons; ātmatvāt — because of being intimately related as the Supersoul; sarva–bhūtānām — of all living entities; siddhatvāt — because of being established; iha — in this world; sarvataḥ — in all directions, in all times and from all angles of vision.
para–avareṣu — in exalted or hellish conditions of life; bhūteṣu — in the living beings; brahma–anta — ending with Lord Brahmā; sthāvara–ādiṣu — beginning with the nonmoving forms of life, the trees and plants; bhautikeṣu — of the material elements; vikāreṣu — in the transformations; bhūteṣu — in the five gross elements of material nature; atha — moreover; mahatsu — in the mahat–tattva, the total material energy; ca — also; guṇeṣu — in the modes of material nature; guṇa–sāmye — in an equilibrium of material qualities; ca — and; guṇa–vyatikare — in the uneven manifestation of the modes of material nature; tathā — as well; ekaḥ — one; eva — only; paraḥ — transcendental; hi — indeed; ātmā — the original source; bhagavān — the Supreme Personality of Godhead; īśvaraḥ — the controller; avyayaḥ — without deteriorating; pratyak — inner; ātma–svarūpeṇa — by His original constitutional position as the Supersoul; dṛśya–rūpeṇa — by His visible forms; ca — also; svayam — personally; vyāpya — pervaded; vyāpaka — all-pervading; nirdeśyaḥ — to be described; hi — certainly; anirdeśyaḥ — not to be described (because of fine, subtle existence); avikalpitaḥ — without differentiation; kevala — only; anubhava–ānanda–svarūpaḥ — whose form is blissful and full of knowledge; parama–īśvaraḥ — the Supreme Personality of Godhead, the supreme ruler; māyayā — by māyā, the illusory energy; antarhita — covered; aiśvaryaḥ — whose unlimited opulence; īyate — is mistaken as; guṇa–sargayā — the interaction of the material modes of nature.
This particular expression in the above verse is also completely reflected in the Shānkaran bhashyas:
That One Supreme Being alone is available as the Atman of all and everything that is experienced as dṛśya, the seen. Therefore, the dṛk and the dṛśya are both the Self alone, implying the sarvam khalvidam brahma theme: (This) Atman is Brahman.// found in the above cited BSB.
This word of the bhagavatam cited above //siddhatvād // too is significantly present in the Taittiriya bhāṣya 1.11.4
नित्यसिद्धात्मदर्शिनः [Those who have the realization of the ever-established Atman.]
The following Brahma sutra bhashya sentences too are about the ‘siddha vastu’ that the Self, that is Brahman is:
किन्तु श्रुत्यादयोऽनुभवादयश्च यथासम्भवमिह प्रमाणम्, अनुभवावसानत्वात् भूतवस्तुविषयत्वाच्च ब्रह्मज्ञानस्य
Shruti, etc. and anubhava, etc. are pranāṇa here, as the context demands. This is because the quest for knowledge culminates in one’s experience of the ever existing Brahman, which alone is the subject matter of that knowledge/experience.
तद्धि असाध्यं नित्यसिद्धस्वभावमेव विद्यया अधिगम्यत इत्यसकृदवादिष्म ।
That (liberation) indeed is incapable of being produced since It is of the nature of being Ever Established.