Sahih Bukhari Volume 005, Book 059, Hadith Number 634.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 59. Military Expeditions Led by the Prophet (PBUH) (Al-Maghaazi)
Narrated By Ibn Abbas : Allah's Apostle said to Muadh bin Jabal when he sent him to Yemen. "You will come to the people of Scripture, and when you reach them, invite them to testify that none has the right to be worshipped except Allah and that Muhammad is His Apostle. And if they obey you in that, then tell them that Allah has enjoined on them five prayers to be performed every day and night. And if they obey you in that, then tell them that Allah has enjoined on them Sadaqa (i.e. Rakat) to be taken from the rich amongst them and given to the poor amongst them. And if they obey you in that, then be cautious! Don't take their best properties (as Zakat) and be afraid of the curse of an oppressed person as there is no screen between his invocation and Allah.