The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
4. (The released soul abides) in non-division (from the highest Self); because that is seen from scripture.
A doubt here arises whether that soul of which the text says, 'Having entered the highest light it manifests itself by its true nature,' remains separate from the highest Self, or abides in the state of non-division from it.--Somebody might be inclined to think that--because in the passage, 'He moves about there,' a distinction is made between the abode and him who abides; and because the clause, 'Having entered the highest light,' mentions an agent and an object (of the agent's activity)--the soul remains distinct from the highest Self.--This view the Sûtra sets aside. The released soul is non-separate from the highest Self.--Why so?--Because
that is seen from scripture. For passages such as 'Thou art that' (Kh. Up. VI, 8, 7); 'I am Brahman' (Bri. Up. I, 4, 10);--Where he sees nothing else' (Kh. Up. VII, 24, 1); 'But there is then nothing second, nothing else different that he could see' (Bri. Up. IV, 3, 23), show that the highest Self abides in the state of non-division. And the fruit must be assumed to correspond to the cognition, according to what was explained under IV, 3, 15. And also such passages as 'Just as pure water poured into pure water remains the same, thus, O Gautama, is the Self of a thinker who knows' (Ka. Up. II, 4, 15), whose object it is to describe the nature of the released soul, declare that there is non-separation only. The same follows from the comparisons (of the soul entering Brahman) to rivers falling into the sea. Passages where separation (of abode and abiding thing, &c.) is expressed, may be explained as, in a secondary sense, expressing non-separation; so e.g. Kh. Up. VII. 24, 1, 'In what does the Infinite rest?--In its own greatness;' and Kh. Up. VII, 25, 2, 'Loving the Self, playing with the Self.'