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. (Using translation from Muslim 19c)
أَنَّ رَسُولَ اللهِ ﷺ لَمَّا بَعَثَ مُعَاذًا عَلَى الْيَمَنِ قَالَ «إِنَّكَ تَقْدُمُ عَلَى قَوْمٍ أَهْلِ كِتَابٍ فَلْيَكُنْ أَوَّلُ مَا تَدْعُوهُمْ إِلَيْهِ عُبَادَةَ اللهِ فَإِذَا عَرَفُوا ذَلِكَ فَأَخْبِرْهُمْ أَنَّ اللهَ ﷻ فَرَضَ عَلَيْهِمْ خَمْسَ صَلَوَاتٍ فِي يَوْمِهِمْ وَلَيْلَتِهِمْ فَإِذَا فَعَلُوا ذَلِكَ فَأَخْبِرْهُمْ أَنَّ اللهَ قَدْ فَرَضَ عَلَيْهِمْ زَكَاةً تُؤْخَذُ مِنْ أَمْوَالِهِمْ فَتُرَدُّ عَلَى فُقَرَائِهِمْ فَإِذَا فَعَلُوا ذَلِكَ فَخُذْ مِنْهُمْ وَاتَّقِ كَرَائِمَ أَمْوَالِهِمْ»