Musnad Ahmed: Hadith 2070

"You are going to some of the People of the Book. Call them to bear witness that there is no god but Allah and that I am the Messenger of Allah. lf they obey you in that, then teach them that Allah, may He be glorified and exalted, has enjoined upon them five prayers each day and night. If they obey you in that, then teach them that Allah, may He be glorified and exalted, has enjoined upon them charity (zakah) to be taken from their rich and given to their poor. If they obey you in that, beware of taking the best of their wealth, and fear the supplication of one who has been wronged, for there is no barrier between it and Allah, may He be glorified and exalted.”