Jami` at-Tirmidhi 625

Ibn Abbas narrated that :the Messenger of Allah sent Mu'adh to Yemen and said to him: "You are going to a people from the People of the Book, so invite them to testify that none has the right to be worshipped but Allah, and that I am the Messenger of Allah. If they comply with that, then inform them that Allah has made five prayers obligatory upon them in a day and a night. If they comply with that, then inform then that Allah has ordained a charity upon their wealth, which is to be taken from the rich among them and given to the poor among them. If they comply with that, then beware of their most precious wealth, and protect yourself from the supplication of the oppressed, for there is no barrier between it and Allah