The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut [1890] at sacred-texts.com

7. If (it is said that the effect is) non-existent (before its origination); we do not allow that because it is a mere negation (without an object).

If Brahman, which is intelligent, pure, and devoid of qualities such as sound, and so on, is supposed to be the cause of an effect which is of an opposite nature, i. e. non-intelligent, impure, possessing the qualities of sound, &c., it follows that the effect has to be considered as non-existing before its actual origination. But this consequence cannot be acceptable to you--the Vedântin--who maintain the doctrine of the effect existing in the cause already.

This objection of yours, we reply, is without any force, on account of its being a mere negation. If you negative the existence of the effect previous to its actual origination, your negation is a mere negation without an object to be negatived. The negation (implied in 'non-existent') can certainly not have for its object the existence of the effect previous to its origination, since the effect must be viewed as 'existent,' through and in the Self of the cause, before its origination as well as after it; for at the present moment also this effect does not exist independently, apart from the cause; according to such scriptural passages as, 'Whosoever looks for anything elsewhere than in the Self is abandoned by everything' (Bri. Up. II, 4, 6). In so far, on the other hand, as the effect exists through the Self of the cause, its existence is the same before the actual beginning of the effect (as after it).--But Brahman, which is devoid of qualities such as sound, &c., is the cause of this world (possessing all those qualities)!--True, but the effect with all its qualities does not exist without the Self of the cause either now or before the actual beginning (of the effect); hence it cannot be said that (according to our doctrine) the effect is non-existing before its actual beginning.--This point will be elucidated in detail in the section treating of the non-difference of cause and effect.