Question 136: What is the Islaamic ruling in your opinion on saying these phrases: “Allaah knows that…,” “Allaah forbid!” “Allaah’s Will,” and “Allaah and His Messenger know best”?
Answer: There is no harm in saying “Allaah knows that…” if the person is saying the truth. There is also no harm in saying “Allaah forbid!” if it is said to seek the removal of something that is causing harm. Saying “Allaah’s Will” is also permissible if it is intended to attribute anything that befalls a person, whether it is illness, poverty, or the like, to Allaah’s Universal Will and Predestination. As for saying “Allaah and His Messenger know best,” this was permissible during the lifetime of the Messenger (peace be upon him), but after his death, we should say “Allaah knows best,” because the Messenger (peace be upon him) does not know what is happening after his death... read more here.