Sahih Muslim | Book 1 | Hadith Number 27

Narrated by Ibn 'Abbas
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 wealth's. Beware of the supplication of the oppressed for there is no barrier between him

and Allah.