The Journal cites "people familiar with the potential deal" for the info. The 50bn cited would be the largest offering in the history of capital markets.
Says the WSJ piece:
after months - long consultations with advisers, the kingdom has decided to host any new Aramco offering on the Riyadh exchange to avoid legal risks associated with an international listing
final decision hasn’t been made on the timing of the deal, but some of the people familiar with the transaction say the kingdom could offer the shares before the end of the year
Aramco has been sounding out potential investors
Notably, the Journal adds that "similar previous plans have fallen through".
The Journal is gated, here is the link if you are able to access it.