It is permissible to say (now) “only Allaah and His Prophet know” in matters that pertain to the unseen, taking into account that the knowledge of the Prophet, sallallaahu alayhi wa sallam, [about the unseen] stopped with his death?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

It is not permissible to say "the Prophet knows best about the unseen" except in what is confirmed that Allaah informed him about. Allaah Says (what means): {[He is] Knower of the unseen and He does not disclose His [knowledge of the] unseen to anyone. Except whom He has approved of messengers, and indeed, He sends before him [i.e. each messenger] and behind him observers.}[Quran 72:26-27]. You have to know that it is only Allaah Who Has the knowledge of the unseen. Allaah Says (what means): {Say, “None in the heavens and the Earth knows the unseen except Allaah, and they do not perceive when they will be resurrected.”}[Quran 27:65].

As regards the Prophet he only knew the unseen that Allaah informed him about. Therefore, what should be said now is exclusively "Allaah Knows best."   

Allaah Knows best.