Hadith: You will come to a people of the Scripture. When you reach them, invite them to testify that none is worthy of worship except Allah and that Muhammad is the Messenger of Allah

عَنِ ابْنِ عَبَّاسٍ رَضِيَ اللَّهُ عَنْهُمَا قَالَ:
قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ لِمُعَاذِ بْنِ جَبَلٍ، حِينَ بَعَثَهُ إِلَى الْيَمَنِ: «إِنَّكَ سَتَأْتِي قَوْمًا أَهْلَ كِتَابٍ، فَإِذَا جِئْتَهُمْ فَادْعُهُمْ إِلَى أَنْ يَشْهَدُوا أَنْ لَا إِلَهَ إِلَّا اللهُ، وَأَنَّ مُحَمَّدًا رَسُولُ اللهِ، فَإِنْ هُمْ أَطَاعُوا لَكَ بِذَلِكَ، فَأَخْبِرْهُمْ أَنَّ اللهَ قَدْ فَرَضَ عَلَيْهِمْ خَمْسَ صَلَوَاتٍ فِي كُلِّ يَوْمٍ وَلَيْلَةٍ، فَإِنْ هُمْ أَطَاعُوا لَكَ بِذَلِكَ، فَأَخْبِرْهُمْ أَنَّ اللهَ قَدْ فَرَضَ عَلَيْهِمْ صَدَقَةً تُؤْخَذُ مِنْ أَغْنِيَائِهِمْ فَتُرَدُّ عَلَى فُقَرَائِهِمْ، فَإِنْ هُمْ أَطَاعُوا لَكَ بِذَلِكَ، فَإِيَّاكَ وَكَرَائِمَ أَمْوَالِهِمْ، وَاتَّقِ دَعْوَةَ الْمَظْلُومِ، فَإِنَّهُ لَيْسَ بَيْنَهُ وَبَيْنَ اللهِ حِجَابٌ».
[صحيح] - [متفق عليه] - [صحيح البخاري: 1496]

Ibn ‘Abbās (may Allah be pleased with him) reported:
The Messenger of Allah (may Allah’s peace and blessings be upon him) said to Mu‘ādh ibn Jabal as he was sending him to Yemen: "You will come to a people of the Scripture. When you reach them, invite them to testify that none is worthy of worship except Allah and that Muhammad is the Messenger of Allah. If they obey you in that, tell them that Allah has enjoined on them five prayers every day and night. If they obey you in that, tell them that Allah has enjoined Zakah on them, to be taken from the rich amongst them and given to the poor amongst them. If they obey you in that, beware of taking their best properties; and fear the invocation of an oppressed person, for there is no barrier between his invocation and Allah.”
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Bukhari - 1496]

Explanation

When the Prophet (may Allah's peace and blessings be upon him) sent Mu‘ādh ibn Jabal (may Allah be pleased with him) to Yemen to call its people to Allah and teach them, he clarified to him that he would meet Christian people and that he should first call them to what is more important and then to what is less important, and so on. He should first call them to reform their creed by testifying that none is worthy of worship except Allah and that Muhammad is the Messenger of Allah. This is because they enter Islam with this testimony. If they complied with that, he should then enjoin them to establish the prayer, for it is the most important obligation after the testimony of Tawhīd (monotheistic belief). If they established the prayer, he should then order the rich amongst them to pay Zakah on their property to the poor amongst them. Thereafter, the Prophet (may Allah's peace and blessings be upon him) warned him not to take their best property; instead, it is required to adopt a moderate approach in this regard. Then, he (may Allah's peace and blessings be upon him) instructed him to avoid oppression, lest an oppressed person might supplicate to Allah against him, as his supplication is readily answered.