It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that when the Prophet (peace and blessings be upon him) sent Mu'adh bin Jabal (may Allah be pleased with him) to Yemen, he (peace and blessings be upon him) said: "O Mu'adh! You are going to the People of the Book. First of all, you are to invite them to the oneness of Allah and the messengership of the Prophet (peace and blessings be upon him). If they accept this, then tell them that Allah has made five prayers obligatory in every day and night. If they accept this too, then tell them: Allah has made sadaqah obligatory on you from your wealth, which will be taken from the wealthy and distributed among the poor. If they accept this, then avoid (taking) their most precious wealth, and beware of the supplication of the oppressed, for there is no barrier between it and Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الصدقات / 13136