Sunan an-Nasa'i 2435

It was narrated that Ibn 'Abbas said:"The Messenger of Allah said to Mu'adh when he sent him to Yemen: 'You are going to some of the People of the book. When you come to them, call them to testify that there is none worthy of worship except Allah and that Muhammad is the Messenger of Allah. If they obey you in that, then tell them that Allah, the Mighty and Sublime, has enjoined on them a charity (Zakah) to be taken from their rich and given to their poor. If they obey you in that, then beware of the supplication of the oppressed person