Narrated Muadh bin Jabal radiallahu anhu:

The Prophet peace be upon him said, "O Muadh! Do you know what Allah's Right upon His slaves is?" I said, "Allah and His Apostle know best." The Prophet peace be upon him said, "To worship Him (Allah) Alone and to join none in worship with Him (Allah). Do you know what their right upon Him is?" I replied, "Allah and His Apostle know best." The Prophet peace be upon him said, "Not to punish them (if they do so).

Narrated Ibn Abbas radiallahu anhu:

When the Prophet peace be upon him sent Muadh to Yemen, he said to him, "You are going to a nation from the people of the Scripture, so let the first thing to which you will invite them, be the Tauhid of Allah. If they learn that, tell them that Allah has enjoined on them, five prayers to be offered in one day and one night. And if they pray, tell them that Allah has enjoined on them Zakat of their properties and it is to be taken from the rich among them and given to the poor. And if they agree to that, then take from them Zakat but avoid the best property of the people."