Sahih Muslim Book - 1, Hadith - 123

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 عَنْ ابْنِ عَبَّاسٍ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ 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.

It was narrated from Ibn 'Abbas that when the Messenger of Allah (s.a.w) sent Mu'adh to Yemen, he said: "You are going to some of the people of the Book, so let the first thing to which you call them be the worship of Allah, the Mighty and Sublime (alone). If they acknowledge Allah (as One), then tell them that Allah has enjoined upon them five prayers to be offered every day and night. If they do that, then tell them that Allah has enjoined on them Zakat to be taken from their wealth and given to their poor. If they accept that, then take it from them, but beware of (taking) the best of their wealth."