by Imam Muslim, translation by Abdul Hamid Siddiqui , Volume: The Book of Faith (Kitab Al-Iman)
Chapter: CALLING PEOPLE TO TESTIFICATIONS AND THE CANONS OF ISLAM
It is reported on the authority of Ibn 'Abbas that Mu'adh said: The Messenger of Allah sent me (as a governor of Yemen) and (at the time of departure) instructed me thus: You will soon find yourself in a community one among the people of the Book, so first call them to testify that there is no god but Allah, that I (Muhammad) am the messenger of Allah, and if they accept this, then tell them Allah has enjoined upon them five prayers during the day and the night and if they accept it, then tell them that Allah has made Zakat obligatory for them that it should be collected from the rich and distributed among the poor, and if they agree to it don't pick up (as a share of Zakat) the best of their wealths. Beware of the supplication of the oppressed for there is no barrier between him and Allah.
It is narrated on the authority of Ibn 'Abbas that when the Messenger of Allah (may peace be upon him) 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 the 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.