Is it permissible to say: "May Allah be upon you"?

The meaning of the phrase "I ask you by God" is to request an action or its abstention in the name of God. There is no religious prohibition against this style, but it is disliked for a Muslim to ask his brother by God due to the potential embarrassment it may cause him, and some scholars have explicitly stated its disliking. One must answer whoever asks by God out of reverence for God.