The Messenger of Allah ﷺ sent Muaz to Yemen and said, "You are going to a people who are People of the Book. Call them to testify that there is no god but Allah and that I am the Messenger of Allah. If they obey you in that, then inform them that Allah has made obligatory upon them five prayers in every day and night. If they obey you in that, then inform them that Allah has made obligatory upon them a charity to be taken from their wealth, collected from their rich and returned to their poor. If they obey you in that, then beware of taking the most prized portions of their wealth, and beware of the supplication of the oppressed, for there is no barrier between it and Allah."
حدثنا أحمد بن حنبل حدثنا وكيع حدثنا زكريا بن إسحق المكي عن يحيى بن عبد الله بن صيفي عن أبي معبد
أن رسول الله ﷺ بعث معاذا إلى اليمن فقال إنك تأتي قوما أهل كتاب فادعهم إلى شهادة أن لا إله إلا الله وأني رسول الله فإن هم أطاعوك لذلك فأعلمهم أن الله افترض عليهم خمس صلوات في كل يوم وليلة فإن هم أطاعوك لذلك فأعلمهم أن الله افترض عليهم صدقة في أموالهم تؤخذ من أغنيائهم وترد على فقرائهم فإن هم أطاعوك لذلك فإياك وكرائم أموالهم واتق دعوة المظلوم فإنها ليس بينها وبين الله حجاب
Bukhari transmitted it in "Injustices," chapter on being wary and cautious of the wronged person's supplication, hadith 2448, in abridged form, and Muslim in "Faith," chapter on inviting people to the two testimonies of faith and the laws of Islam, hadith 19, from the hadith of Waki through the same chain.