Abdullah bin Abbas reported,
When the Messenger of Allah ﷺ sent Muaz bin Jabal to Yemen, he said to him, "You are going to a people who are People of the Book. Call them to bear witness 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 five prayers obligatory upon them in every day and night. If they obey you in that, then inform them that Allah has made charity obligatory upon them, taken from their wealthy and returned to their poor. If they obey you in that, then beware of taking the choicest of their wealth, and beware of the supplication of the wronged, for there is no veil between it and Allah ."
Ahmad 2071
Sahihحدثنا وكيع حدثنا زكريا بن إسحاق المكي عن يحيى بن عبد الله بن صيفي عن أبي معبد
عن ابن عباس
أن رسول الله ﷺ لما بعث معاذ بن جبل إلى اليمن قال إنك تأتي قوما أهل كتاب فادعهم إلى شهادة أن لا إله إلا الله وأني رسول الله فإن هم أطاعوك لذلك فأعلمهم أن الله عز وجل افترض عليهم خمس صلوات في كل يوم وليلة فإن أطاعوا لذلك فأعلمهم أن الله عز وجل افترض عليهم صدقة في أموالهم تؤخذ من أغنيائهم وترد في فقرائهم فإن هم أطاعوك لذلك فإياك وكرائم أموالهم واتق دعوة المظلوم فإنها ليس بينها وبين الله عز وجل حجاب
Its isnad is sahih. Yahya bin Abdullah bin Sayfi, also said to be Yahya bin Abdullah bin Muhammad bin Yahya bin Sayfi, and other names are also given for him, is trustworthy. He was deemed trustworthy by Ibn Main, Nasai, and Ibn Saad, and Ibn Hibban mentioned him in Al-Thiqat. Bukhari gave his biography in Al-Kabir 2/4/284. The hadith was narrated by the authors of the six books, as noted in Dhakhair Al-Mawarith 2952."The finest of their wealth" — in Al-Nihayah it is explained as meaning the most valuable and prized possessions, those to which their owner's heart is attached and which he singles out for himself, on account of their being, so far as possible, the most excellent of what belongs to him. Its singular form is "karimah."