Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that when the Prophet (peace and blessings be upon him) sent Sayyiduna Mu'adh (may Allah be pleased with him) to Yemen, he said: "You are going to a people who are People of the Book. So the first thing you invite them to should be the worship of Allah, Glorified and Exalted. When they come to know Allah, then tell them that Allah Almighty has made five prayers obligatory upon them in each day and night. When they do that, inform them that Allah has also made zakah obligatory upon them, which will be taken from the wealthy and given to the poor. When they obey this, take it from them, and beware of the choicest of their wealth."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7303