It was narrated from Ibn 'Abbas that the Prophet P.B.U.H nt Mu 'aadh. to Yemen, and said":You are going to some people among the People of the Book. CalI them to bear witness that none has the rIght to be worshiped but Allah, and that I am the Messenger of Allah P.B.U.H. they obey that, then tell them that Allah has enjoined upon them live prayers every day and night. If they Obey that, then telI them that Allah has enjoined upon them charity (Zakat) from their wealth, to be taken from their rich and given to their poor. If they obey that, then beware of (taking) the best of their wealth. And beware of the supplication of the oppressed, for there is no barrier between it and Allah." (Sahih)