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

16. And on account of that which is posterior (i. e. the effect) being that which is.

For the following reason also the effect is to be considered as non-different (from the cause). That which is posterior in time, i.e. the effect, is declared by Scripture to have, previous to its actual beginning, its Being in the cause, by the Self of the cause merely. For in passages like,' In the beginning, my dear, this was that only which is' (Kh. Up. VI, 2, 3); and, 'Verily, in the beginning this was Self, one only' (Ait. Âr. II, 4, i, i), the effect which is denoted by the word 'this' appears in grammatical co-ordination with (the word denoting) the cause (from which it appears that both inhere in the same substratum). A thing, on the other hand, which does not exist in another thing by the Self of the latter is not produced from that other thing; for instance, oil is not produced from sand. Hence as there is non-difference before the production (of the effect), we understand that the effect even after having been produced continues to be non-different from the cause. As the cause, i e. Brahman, is in all time neither more nor less than that which is, so the effect also, viz. the world, is in all time only that which is. But that which is is one only; therefore the effect is non-different from the cause.