It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that when the Messenger of Allah (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 let the first thing you invite them to be the worship of Allah. When they come to know that, tell them that Allah, the Mighty and Majestic, has made five prayers obligatory upon them in every day and night. When they do that, tell them that Allah has made zakah obligatory upon them, to be taken from their wealth and returned to their poor. When they do that, collect the zakah from them, and beware of taking the best of their wealth."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12155