Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said to Mu'adh bin Jabal (may Allah be pleased with him) when he sent him to Yemen: "You will come to a people of the People of the Book. When you come to them, invite them to testify that there is no god but Allah and that Muhammad (peace and blessings be upon him) is the Messenger of Allah. If they obey you in that, inform them that Allah has made five prayers obligatory upon them in every day and night. If they obey you in that too, inform them that Allah has made zakah obligatory upon them, to be taken from their rich and given to their poor. If they accept that from you too, then beware of their choicest wealth, and beware of the supplication of the oppressed, for there is no veil between the oppressed and Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7276