Sayyiduna Ibn Abbas (may Allah be pleased with them both) said that when the Messenger of Allah (peace and blessings be upon him) sent Mu'adh bin Jabal (may Allah be pleased with him) to Yemen, he said: "You are going to a people who are People of the Book. The first thing to which you call them should be the Oneness of Allah. When they have come to know that, tell them that five prayers have been made obligatory upon them in a day and a night. When they start praying, tell them that zakah has also been made obligatory in their wealth, which will be taken from the wealthy among them and distributed among the poor. When they accept that as well, take the zakah from them, but avoid the choicest of their wealth."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الصدقات / 13112