Sahih Muslim 19 c
It is narrated on the authority of Ibn 'Abbas that when the Messenger of Allah (ﷺ) sent Mu'adh towards Yemen (as governor) he said to him:
Verily you would reach a community of the people of the Book, the very first thing to which you should call them is the worship of Allah, may His Glory be Magnificent, and when they become fully aware of Allah, instruct them that He has enjoined five prayers on them during the day and the night, and when they begin observing it, then instruct them that verily Allah has made Zakat obligatory for them which would be collected from the wealthy amongst them and distributed to their needy ones, and when they submit to it, then collect it from them and avoid (the temptation) of selecting the best (items) of their riches.
حَدَّثَنَا أُمَيَّةُ بْنُ بِسْطَامَ الْعَيْشِيُّ، حَدَّثَنَا يَزِيدُ بْنُ زُرَيْعٍ، حَدَّثَنَا رَوْحٌ، - وَهُوَ ابْنُ الْقَاسِمِ - عَنْ إِسْمَاعِيلَ بْنِ أُمَيَّةَ، عَنْ يَحْيَى بْنِ عَبْدِ اللَّهِ بْنِ صَيْفِيٍّ، عَنْ أَبِي مَعْبَدٍ، عَنِ ابْنِ عَبَّاسٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم لَمَّا بَعَثَ مُعَاذًا إِلَى الْيَمَنِ قَالَ ‏
"‏ إِنَّكَ تَقْدَمُ عَلَى قَوْمٍ أَهْلِ كِتَابٍ فَلْيَكُنْ أَوَّلَ مَا تَدْعُوهُمْ إِلَيْهِ عِبَادَةُ اللَّهِ عَزَّ وَجَلَّ فَإِذَا عَرَفُوا اللَّهَ فَأَخْبِرْهُمْ أَنَّ اللَّهَ فَرَضَ عَلَيْهِمْ خَمْسَ صَلَوَاتٍ فِي يَوْمِهِمْ وَلَيْلَتِهِمْ فَإِذَا فَعَلُوا فَأَخْبِرْهُمْ أَنَّ اللَّهَ قَدْ فَرَضَ عَلَيْهِمْ زَكَاةً تُؤْخَذُ مِنْ أَغْنِيَائِهِمْ فَتُرَدُّ عَلَى فُقَرَائِهِمْ فَإِذَا أَطَاعُوا بِهَا فَخُذْ مِنْهُمْ وَتَوَقَّ كَرَائِمَ أَمْوَالِهِمْ ‏"‏ ‏.‏