The claim is that, according to Phaedo 92d, Plato is thinking of the soul as having within it some sort of 'being' (ousia) which is such that, (i) to affirm the independent existence of the Forms, regarding this 'being' as derived from them, and (ii) to affirm the existence of the soul before its entering the body, amount to the same thing.
Here are two passages which support this interpretation, both of which are plausibly taken as antecedent referents.
In the first, 75d, Socrates compares our applying the label 'what is' to the Forms, to our placing a seal or guarantee of authenticity upon them. This suggests that 'what is' is applied to the Forms as to something primary; that it is not applied to them derivatively. (There is no suggestion in the Phaedo that there is a primary form of Existence Itself from which other Forms besides that derive their existence. The dialogue seems to recognize no hierarchy in the Forms.) If so, then the ousia in 92d, to which the term 'what is' is said to apply derivatively, should not be understood as the ousia of the Forms.
“Now if we had acquired that knowledge before we were born, and were born with it, we knew before we were born and at the moment of birth not only the equal and the greater and the less, but all such abstractions? For our present argument is no more concerned with the equal than with absolute beauty and the absolute good and the just and the holy, and, in short,
[75d]with all those things which we stamp with the seal of absolute in our dialectic process of questions and answers; so that we must necessarily have acquired knowledge of all these before our birth.”
ou)kou=n ei) me\n labo/ntej au)th\n pro\ tou= gene/sqai e)/xontej e)geno/meqa, h)pista/meqa kai\ pri\n gene/sqai kai\ eu)qu\j geno/menoi ou) mo/non to\ i)/son kai\ to\ mei=zon kai\ to\ e)/latton a)lla\ kai\ su/mpanta ta\ toiau=ta; ou) ga\r peri\ tou= i)/sou nu=n o( lo/goj h(mi=n ma=llo/n ti h)\ kai\ peri\ au)tou= tou= kalou= kai\ au)tou= tou= a)gaqou= kai\ dikai/ou kai\ o(si/ou kai/, o(/per le/gw, peri\ a(pa/ntwn oi(=j e)pisfragizo/meqa to\ “au)to\ o(\ e)/sti” kai\ e)n tai=j e)rwth/sesin e)rwtw=ntej kai\ e)n tai=j a)pokri/sesin a)pokrino/menoi. w(/ste a)nagkai=on h(mi=n tou/twn pa/ntwn ta\j e)pisth/maj pro\ tou= gene/sqai ei)lhfe/nai.
In the second, 76e, Socrates actually identifies the ousia of the Forms with something in the soul:
“Then, Simmias” said he, “is this the state of the case? If, as we are always saying, the beautiful exists, and the good, and every essence of that kind, and if we refer all our sensations to these,
[76e]which we find existed previously and are now ours, and compare our sensations with these, is it not a necessary inference that just as these abstractions exist, so our souls existed before we were born; and if these abstractions do not exist, our argument is of no force? Is this the case, and is it equally certain that provided these things exist our souls also existed before we were born, and that if these do not exist, neither did our souls?”
a)=r' ou)=n ou(/twj e)/xei, e)/fh, h(mi=n, w)= Simmi/a; ei) me\n e)/stin a(\ qrulou=men a)ei/, kalo/n te/ ti kai\ a)gaqo\n kai\ pa=sa h( toiau/th ou)si/a, kai\ e)pi\ tau/thn ta\ e)k tw=n ai)sqh/sewn pa/nta a)nafe/romen, u(pa/rxousan pro/teron a)neuri/skontej h(mete/ran ou)=san, kai\ tau=ta e)kei/nh| a)peika/zomen, a)nagkai=on, ou(/twj w(/sper kai\ tau=ta e)/stin, ou(/twj kai\ th\n h(mete/ran yuxh\n ei)=nai kai\ pri\n gegone/nai h(ma=j: ei) de\ mh\ e)/sti tau=ta, a)/llwj a)\n o( lo/goj ou(=toj ei)rhme/noj ei)/h; a)=r' ou(/twj e)/xei, kai\ i)/sh a)na/gkh tau=ta/ te ei)=nai kai\ ta\j h(mete/raj yuxa\j pri\n kai\ h(ma=j gegone/nai, kai\ ei) mh\ tau=ta, ou)de\ ta/de;
Rowe remarks on this second passage: "What 'was there before' (u(pa/rxousan pro/teron), is 'rediscovered' (a)neuri/skontej), and 'is ours' (h(mete/ran ou)=san), is clearly not the ou)si/a in question, as the text literally says, but our knowledge of it." But if knowledge of a Form amounts to contact, presence, or reception, as Socrates variously says, then the difference between our knowledge of the Forms, and the Forms themselves, is unimportant and makes this manner of speech understandable.