What is the legitimacy of saying to someone "Be the hand of God," meaning His mercy, power, and will, to encourage doing good?

First: It is not permissible to say to a created being, "Be the hand of Allah, the Bestower of Goodness," because nothing should be attributed to Allah Almighty or ascribed to Him except with a textual religious proof. Also, this expression might suggest that "the hand of Allah" means power or strength, whereas it is a real attribute of Allah Almighty, as unanimously agreed upon by the Ahl al-Sunnah (Sunnis).

Second: As for the saying, "Be the mercy of Allah to His creation," there is no objection to it, because it means that a person should be merciful and benevolent towards people. In doing so, he becomes part of Allah's created mercy that He has placed among His servants, which is one of the hundred mercies that Allah created and sent one of them to His creation, as stated in the Prophetic hadiths. It is advisable to avoid such ambiguous phrases to prevent ignorance and misunderstanding.