Is the saying "Allah and His Messenger know best" correct, knowing that Allah alone knows best and that the knowledge of the Messenger is from what Allah taught him?

The saying "Allah and His Messenger know best" is correct in matters of Sharia (Islamic law), because the Prophet, may Allah bless him and grant him peace, was the most knowledgeable of people concerning the Sharia of Allah. This does not mean that his knowledge is like the knowledge of Allah. As for matters outside of Sharia, it is not permissible to say it, because the Messenger, may Allah bless him and grant him peace, did not know the unseen during his lifetime, so how could he know it after his death?